In recent years, with changes in the strategic environment, the United States has reflected on the long-term grand strategy it has been implementing, and made some profound adjustments by changing its strategic thinking and adopting new strategic measures, so as to maintain a balance between strategic means and objectives and its leading role in the world. In this process, scholars in the US have conducted a series of heated debates on the grand strategy, and on the nature and scope of the United States’ engagement in global affairs, which has had some impact on its policies. By analyzing the changes in the strategic environment, as well as the changes in the United States’ power and position, this article presents and analyzes the process and content of the US’ adjustments to its grand strategy, with an aim of understanding clearly what the nature of its grand strategy is now and the direction it will take in the future, and evaluating its impact on the regional and international situation and on China-US relations.
　　Background of United States’ Grand Strategy Adjustments
　　Since the end of the Cold War, the United States, as an established power, has been pursuing an expansionary grand strategy and involving itself in international affairs in a broad and profound manner so as to maintain its dominant position in the international system. However, in the 21st century, this strategy has failed to bring the outcomes the US desired. Instead, it has become a strategy of over-expansion, and has brought many negative results, such as the weakening of the US leadership, as well as harming regional order and undermining international norms. The US economy has been hit badly by the international financial crisis. But during the same period, China, India and other rising powers have maintained a momentum of growth, which has led to the US undergoing some adverse changes in terms of its relative power. In this context, the US has to adjust its grand strategy.
　　Objectives of the United States’ Grand Strategy
　　A grand strategy is a matched relationship between a nation’s core objectives and the various power resources and instruments it can use to achieve those objectives. A country must first identify its objectives and interests, then identify if there are any challenges or threats it faces to achieving them, and then choose specific instruments to tackle those challenges and realize the desired objectives. Therefore, a grand strategy is a conceptual roadmap that depicts how goals are identified and prioritized, and how to match available resources with national interests. An effective grand strategy is a good match between objectives and instruments. That means a country’s commitments should not be too large and beyond its capacity; otherwise there will be many negative impacts and even risks. 　　The central goal of the grand strategy of the United States is to maintain its dominant position in the international system and to maintain its security, prosperity and freedom. To that end, the US needs to achieve the following three objectives: to shape an external security environment in order to reduce the short-and-middle-term threats to its security; to spread the model of a free market economy worldwide so as to promote its economic prosperity; and to build and maintain an international institutional system that is conducive to international cooperation in favor of the US. For a long time, in order to achieve the above objectives, the US has pursued an expansionary, globally engaged grand strategy, strengthened its forward-deployed military presence and provided security assurances to its allies, and claimed to promote democracy and protect human rights throughout the world on the basis of its superior military capabilities and available power.
　　Under the premise of ensuring its dominance in the Americas, the US grand strategy places particular emphasis on Europe, East Asia and the Middle East/Persian Gulf – three core regions in the world, and it invests many resources in these strategic priorities. In Europe and East Asia, the main concern of the US is to maintain the regional balance of power, to prevent the emergence of any regional hegemony, and restrain Russia and China, the two most powerful countries in the region. In the Middle East/Persian Gulf region, its goal is a) to prevent the emergence of a regional hegemony, maintain the order of oil exporters in the region and preserve economic prosperity for itself and the world at large; and b) to protect its ally Israel, the pivotal country in its Middle East strategy, and ensure its security. In short, the US grand strategy is committed to maintaining its dominant position in the world, and preventing a potential regional hegemony from being established in Europe, East Asia or the Persian Gulf region.
　　Changes in the United States’ Relative Power
　　Since the financial crisis erupted in 2008, the United States’ dominance in the international order has declined, and claims that the US has started to decline have been frequently heard. To what extent is that correct? We have to distinguish between two different types of great power decline before reaching an answer. One is absolute decline, which means a country declines compared with its peak power. The other is relative decline, which means a country has seen its strength over others narrowing. Professor Joseph Nye of Harvard University believes that the key to understanding the concept of “decline” is to think about the relative position of the United States in the world, and to consider relative decline instead of absolute decline. He further clarifies “decline” in terms of both external power and domestic decay. The former is seen compared with other nations in the international system while the latter represents a lack of an internal capacity to change resources into power. 　　To start with, let us look at the external strength of the United States. Strength is multidimensional, changing and difficult to measure accurately. But in the long term, the most important indicators of a country’s relative strength to others are economic power and military capability. In terms of its economic strength, the United States has seen its advantages lessened due to economic growth of China, India and other emerging powers. In 2015, China’s GDP accounted for 15.5% of the world GDP, 63.4% of the United States’ GDP, an increase of 11% compared with that of 2012. In terms of military strength, the US still has a great advantage. Its military spending is far larger than that of other countries. It boasts the world’s most modern weapons and equipment, maintains sea, air, and even outer space and cyberspace supremacy, and has military allies all over the world. But the US is also facing some challenges. Changes to other countries’ military strategies and technology are eroding its advantages. Anti-ship cruise missiles make it more difficult for the US to get close to a coast and advanced surface-to-air missiles cost it more to maintain air supremacy. In addition, although its military budget is still the largest in the world, the US has kept cutting its military spending in recent years. Between 2010 and 2016, the United States’ defense budget shrank 14% in real terms, and its proportion in the total GDP dropped to about 30%. Second, we should assess whether the United States is experiencing a political decline. According to Francis Fukuyama, the US is experiencing political decay, a combination of the constitutional system featuring separation of powers, polarized political infighting and financially strong interest groups that has resulted in “veto politics” in the US and helped create a situation in which the government is prevented from doing anything. Given the criticism of the Republicans and the congressional constraints on the Obama administration, as well as the chaos in the 2016 US presidential election, the current US political system can be seen to have declined to a certain degree. Clearly this will negatively impact the capacity of the United States to change resources into strength.
　　Economic scale alone cannot measure the strength of a country in the international system. Factors such as its economic structure, technology and military strength must also be taken into account. As one power that is closest to the United States with respect to overall capability, and one considered most likely to challenge the US hegemony, China still lags far behind the US in terms of these indicators. As Thomas J. Christensen points out, the rise of China is real, and it has attracted the attention of so many observers and leaders around the world. However, China’s economic, political and military might is often exaggerated and the significance of its rise misread. To conclude, the United States has not suffered an absolute decline, but only seen some unfavorable changes in its relative strength. Notably, the speed and degree of these changes are moderate. Whether in terms of hard power, like its economy and military, or in terms of soft power such as culture and values, the US is still in a dominant position in the current international system. 　　Overexpansion of the United States
　　Overexpansion refers to a situation where a hegemony pursues strategic objectives that far exceed its capacities and whose strategic costs break the cost-benefit equilibrium point, thus leading to a strategic dilemma. Under the George W. Bush administration, based on the strategic concept of “liberal interventionism,” the United States, under the banner of its “War on Terror” and to the fullest extent of “unilateralism” and “interventionism,” launched the protracted and expensive Afghan and Iraqi wars. However, these wars failed to make Afghanistan and Iraq models of democracy for the rest of the Middle East countries, and instead worsened the regional turmoil and lead to birth of terrorist forces such as the “Islamic State” (ISIS). The US overreached itself and it could hardly bear the burden of its heavy military spending. Its domestic financial deficit and debt crisis worsened. There was a rising anti-war sentiment and more and more people called for the troops to “come home” (to leave and retrench). The US not only paid high bills (estimated at between $4-6 trillion) for these wars, but its image and reputation were also tarnished due to the following humanitarian crises and the scandal of prisoners being tortured, which harmed the legitimacy foundations for its interference in international affairs. In addition, the long-term quagmire in the Middle East also resulted in an imbalance in the United States’ global strategy, and the attention it gave to the Asia-Pacific and its investment in the region were relatively inadequate given its aims. As the world’s focus shifted from the Atlantic to the Asia-Pacific region, and combined with a changing geopolitical landscape in the region brought by China’s continuous and rapid rise, the United States needed to invest more resources and attention in the Asia-Pacific to maintain its dominance and influence in the region.
　　Debate on the United States’ Grand Strategy
　　The United States has two options for adjusting its grand strategy: deep engagement or retrenchment. The former seeks to continue the existing expansionary grand strategy: maintaining a forward-deployed military presence abroad, fulfilling international security commitments, promoting the values of freedom and democracy, conducting humanitarian interventions and seeking regime changes in other countries, but based on some appropriate changes and innovations, by means of increasing resources, such as increasing domestic taxes to fill budgetary gap, or asking its allies to provide more financial support. Supporters of deep engagement (“deep engagers”) believe that deep engagement is an effective way for the United States to pursue its core interests such as security, prosperity and freedom, and its costs should not be exaggerated nor its benefits underestimated. Given the fact that the US will remain the only superpower for the next few decades, “deep engagers” argue it is its best strategic choice to continue deep engagement. In contrast, supporters of the retrenchment strategy argue that the United Sates pays a high price to pursue the deep engagement strategy. They say the cost to maintain the alliance system and fulfill its security commitments is far greater than the benefits. And its military intervention abroad creates more problems than those it helps resolve. Therefore, they argue, the US should not keep pursuing an expansionary grand strategy. Instead it should make substantial adjustments to its existing strategy, reduce military presence overseas and commitments to allies, carry out strategic restraint and contraction by reducing its commitments, cutting spending, lowering risks and transferring the burden, and putting its resources and strategic priority into domestic challenges, such as boosting domestic economy and coping with China’s economic rise, which they say are more important. 　　The intense domestic debate over the United States’ grand strategy reflects different perceptions in the country about its own strategic environment and interests, goals, roles and means in that environment. The focus of the debate is around a series of key questions, for instance: “Can the US economy continue to afford the grand strategy of deep engagement?” “Does deep engagement fit today’s international landscape and geopolitical changes?” “What role should the US play in the international security affairs?” and “Should the US be committed to promoting the values of freedom and democracy?”
　　Can US domestic budget continue to afford deep engagement?
　　Proponents of retrenchment argue that, under the current or expected economic situation, the United States can hardly afford the expensive costs of deep engagement. The US economy was hit badly by the financial crisis. The great pressures the government is under to pay for social welfare, and the rising fiscal deficits and debts, all contribute to limiting US’ capability to pay for overseas operations. Most nations, usually out of financial constraints, prioritize investment in transportation, education, pensions, healthcare and other domestic projects, and limit their outbound investment. But the US, thanks to its strong national strength, was previously not limited in this way. However, that era is ending and the new era demands the United States cut its defense and diplomatic spending, and transfer resources to boost its domestic economy.
　　Deep engagers believe that, despite the United States’ serious budgetary problems and its slow economic recovery, it cannot and should not resolve its fiscal crisis through a grand strategy of retrenchment, and believe that the current level of defense and diplomatic spending is still affordable. They put forward the following two arguments: First, the financial crisis did not stem from the increase in defense spending, which is smaller than spending on domestic social welfare. For example, the United States’ defense spending accounted for 16% of the federal budget in 2015, and that share is declining; in the same year, spending on domestic social welfare accounted for 49% of the federal budget, and the share is still rising. Historically, the US’ defense spending during the Cold War period was much higher than the current level. Between 1950 and 1990, the annual defense spending by the US accounted for 7.6% of its GDP on average, and it dropped to less than 5% after the end of the Cold War. That figure did not rise above 5% afterwards even in the peak spending period of the Afghan and Iraqi wars. Therefore, the solution to the current fiscal crisis is to change the unsustainable trend of domestic social welfare programs. Second, the United States could save about 1% of its GDP by reducing its international commitments. But these will be offset by much higher spending when the US restarts overseas operations in pursuit of its core interests. Therefore, economically speaking, compared to huge losses brought by withdrawing from overseas, the current practice of investing resources and maintaining a military presence overseas is cost-effective. 　　Does deep engagement fit international landscape and geo-political changes?
　　Proponents of retrenchment argue that deep engagement is not suitable for the multipolarization of the international system, which is currently underway. The United States is no longer the only superpower. The center of the world is shifting from the Atlantic to the Asia-Pacific region. Therefore, it is necessary for the US to adjust its grand strategy, retrench at the global level and redefine the priorities in its diplomacy. Deep engagement will provoke two kinds of resistance against the US. First, it will make other countries counterbalance the strength of the US by means of alliances, internal balance (changing potential into military capacity), or “soft checks and balances” (institutions, norms or other non-military means). Second, overexpansion will lead to hegemony decline. As Paul Kennedy pointed out in The Rise and Fall of Great Powers, the US will expand under the temptation of hegemony and end up declining as a result of overexpansion. Therefore, the US should retrench in time to avoid the historical destiny of a hegemonic country.
　　Deep engagers believe that since the United States will still be the only superpower for the next few decades, deep engagement is still the best choice to serve its interests. They do not deny that the US has suffered a relative decline in such areas as economic strength in recent years. But they stress that it still maintains an obvious advantage in many other areas, for example in military, science and education. All these relative strengths enable the US to continue to lead the international system. Deep engagement will not bring an alliance to counterbalance the US since the counterbalance measures will only be taken by a country against the geographically close hegemony or state which poses the largest threat to it. The US is neither of these: the Pacific and Atlantic oceans detach it from the rest of the world; and it has no ambition to occupy the territory of other countries, which weakens other countries’ sense of being threatened by the US.
　　What role should the US play in international security affairs?
　　Proponents of retrenchment hold that it is not wise to fight for military supremacy in every region of the world and the United States should aim to maintain the balance of power, rather than being the dominant power. With its comparative military advantages, especially in navy and air force, the US should maintain the regional balance of power mainly through strengthening its air and sea capabilities instead of sending ground forces into battle. It should abandon its ambitions of seeking regime changes and transforming other countries, because the pursuit of ideological goals will embroil the US in Crusade-style wars that are not conducive to its interests. These conflicts are not only a waste of energy and resources that could otherwise be used to boost domestic growth, but also inciters of international mistrust, unrest and humanitarian crises, which could lead to more terrorist attacks against the US and its allies. Conflicts in Afghanistan, Iraq, Libya and Syria serve as such examples. Expanding democracy, they argue, sometimes requires military occupation and interference in local political arrangements, which will invariably arouse the hatred of local nationalists, who tend to turn to terrorism because of their inability to confront the US directly. The wars in Afghanistan and Iraq launched by the US broke the balance of power in the Middle East, cost the US thousands of lives and spawned the terrorist organization Islamic State. As Barack Obama said, “Almost every great world power has succumbed to overextension. What I think is not smart is the idea that every time there is a problem, we send in our military to impose order. We just can’t do that.” In short, for the supporters of retrenchment, the US should reduce its external commitments, reduce its number of troops abroad, and intervene to restore the balance of power only when the regional balance is broken. 　　Deep engagers believe that the United States should continue to invest its resources and maintain a global military presence in order to complete a series of different tasks: defend its territories, ensure free access to the ocean, air, space, cyberspace and other global commons, maintain peace in Europe, try to build peace in the Greater Middle East region, and cope with the rise of emerging powers in the Asia-Pacific region. The US military should be able to deter an enemy and potential aggressors, and make its allies and partners believe that the US is capable of providing them with security support. The alliance systems the US has built in North America, Europe and East Asia are a lasting source of its strategic advantage and influence, and ultimately help strengthen its own security. Therefore, it should firmly maintain its security commitments to its allies. The implementation of the rebalancing strategy to the Asia-Pacific should not be pursued at the expense of US security commitments to Europe, the Middle East or any other region. The US security commitments and forward-deployed military will not only prevent an ambitious regional hegemony from expanding, but also restrain its allies and partners from taking provocative actions, thus avoiding regional arms races and worsening its security dilemmas.
　　Should the US be committed to promoting liberal democratic values?
　　Retrenchment proponents argue that to promote liberal democratic values deviates from those core missions that advance American values. The United States should stop the practice and make policies, domestic and foreign, according to its national interests. It is not an essential goal and the obsession with it can have repercussions for US national security and cost valuable resources in other countries to carry out the so-called “nation-building” tasks that are difficult to accomplish. The US has been mired in prolonged and costly wars in Iraq and Afghanistan because it has attempted to transplant its liberal democratic values in these two countries, without knowing that this is a wrong idea, one that has resulted in the US failure to distinguish significant security interests and marginalized interests.
　　Deep engagers hold that to promote liberal democracy around the world is necessary to achieve the geopolitical goals of the United States, and can increase its strength and global influence. Moreover, the United States’ closest and reliable allies are democracies. Therefore, the promotion of democracy will expand the range of countries that can establish lasting and close relations with the US. The US should continue to be a strong proponent of democratic politics and the free market economy, and a champion of human rights that is firmly opposed to any human rights abuses. Expanding American values promotes human freedom and dignity and is thus morally justified. It is also a core security interest of the United States, since a free and democratic world will be a safer world, which is conducive to US security. 　　Why Different Choices in Grand Strategy
　　The debate on grand strategy reflects the differences within the United States in three areas:
　　Understandings on current US strength and development trend
　　For a long time, the global dominance of the United States was taken for granted, but it is now a matter of debate in both policymaking and academic communities. Some people are optimistic that the relative decline that the US is going through is only temporary, and say the US is still in the leading position in various dimensions; there are no competitors to its hegemony; the complex international situation is also conducive to the US retaining its dominance; and the 21st century is still an “American century.” Therefore, it should continue to implement its grand strategy of deep engagement and should not make any fundamental policy changes.
　　Pessimists argue that based on the theory of “hegemony transition” and the “rise and fall of great powers” in international politics, with the rise of emerging powers such as China and India, the decline of the United States is inevitable; the current US economy can hardly support the burden of its global deployments, and its geopolitical influence will be gradually weakened. Therefore, the US should reposition its own role in the world through strategic contraction and other policies, to prolong its hegemony or seek a decent decline.
　　Both the optimists and the pessimists look at China as an important reference for observing the relative power of the United States. Optimists believe that although the economic gap between China and the US is shrinking, it remains large in the two major power indicators of science and technology and military strength. China is at its best rising to be a superpower, but there is still a long way to go before it can catch up with the US. The pessimists, however, argue that the rise of China poses the principal strategic challenge to the US. China is expected to overtake the US as the world’s largest economy in the next 10 to 15 years; even if the rise of China’s economic, military and geopolitical influence slows in the future, the world will still witness in the next few decades the largest power transfer since the late 19th and early 20th century when the rise of the US started. In the view of pessimists, China’s Belt and Road Initiative and its proposal of establishing the Asian Infrastructure Investment Bank, as well as the International Monetary Fund’s inclusion on the renminbi in its Special Drawing Rights basket, are notable examples of China’s rise. Different understandings of how powerful the US is and the different perspectives on China’s rise are important reasons why there are different proposals for the US grand strategy within the United States. 　　Two schools of thought dominating US diplomatic strategy
　　There are two schools of thought that shape American diplomacy: idealism and pragmatism. Both shape the grand strategy of the United States. Idealism, or liberalism, has existed since the founding of the United States. It holds that the US shoulders the mission of imparting “freedom and democracy” to the world, and the US has the “manifest destiny” to lead and save the world. At the same time, it holds that a world of democracies will be peaceful and stable, and the US security and interests will be safest in an open world made up of ideologically similar nations. The American way of influencing the world should be through activism, and it has the responsibility to protect the values of freedom and democracy and promote democracy and human rights in other countries. In this way, idealism has shaped an expansive grand strategy that has led the United States to overexpansion and liberal imperialism, triggered the engagement policy in US foreign policy, and even launched some avoidable foreign wars. In the current grand strategy debate, the idealists or liberals consider the US to have a moral and strategic need to promote freedom and protect human rights; the expansion of democracy, they argue, will make the world largely free of war and brutality, ease suffering and maintain the United States’ security.
　　Realism began with the farewell speech of the founding father, President George Washington, and for a long time afterwards, it was manifested by isolationism. After World War II, isolationism in the United States almost vanished. In recent years, however, the frustration about the wars in Afghanistan and Iraq, coupled with the blow of the financial crisis, has brought a resurgence of isolationist sentiment in the US. It has translated into a realist position in the current grand strategy debate. Obama claimed to be a realist, repeatedly claiming since taking office that the United States needed to focus on domestic development. He argued that the US should exercise restraint, not arbitrary external interference. Interference in external affairs should come only when it can really make a difference. In the 2016 presidential election, the Republican nominee Donald Trump’s foreign strategy and policy preferences show an isolationist tendency, and they have attracted a large number of supporters. A poll by the Pew Research Center in April 2016 found that 57% of Americans thought the US should handle its own problems and let other countries deal with their problems. The realists are skeptical about the exceptionalism of the United States and the unique leadership role of the US in defending freedom, democracy and human rights. They argue that the expansion of democracy by threat of force can hardly work, and will damage the image and reputation of the US as well as its own values. The US should proceed from the realities in the balance of forces and geopolitics and formulate a foreign policy that serves its national interests. It should not be obsessed with idealist goals that put its own interests and national security at risk. 　　Notions on how to safeguard US core interests
　　It is argued that the geopolitical environment of the United States has a unique advantage: its northern and southern neighbors are relatively weak and hardly pose a threat and the Atlantic to the east and the Pacific to the west are natural safety barriers. In addition, the US has a vast land, rich natural resources and a large vibrant population. All these have enabled the United States to develop into the world’s largest economy and the most powerful military power with its own resources. It also has thousands of nuclear weapons, reducing the possibility of other countries launching attacks on US soil. Therefore, even without a costly expansive grand strategy, the United States will be able to maintain its strength and security.
　　But there is another view that the security, prosperity and freedom of the United States are closely related to the open international system, and that the maintenance of an open international system requires the existence of a hegemonic state that provides public goods, including global order, cooperation, security, exchange rate stability, etc.. This is the so-called “hegemonic stability” theory. According to this theory, the US is the only country capable and willing to provide the necessary public goods for a stable and open international system; therefore, the US needs to implement a grand strategy of deep engagement, even through foreign military intervention when necessary.
　　Impact of the Grand Strategy Debate
　　The grand strategy debate has provided a strategic option to policymakers, and has certain impact on the US government’s strategic adjustments in recent years. It may continue to shape the future of the US strategic adjustments.
　　First, the grand strategy debate is changing public opinion in the United States, the political ecology, and thus affecting policy adjustments. As a result of the financial crisis, the relative strength of the US has declined. In addition, the protracted wars in Afghanistan and Iraq have been expensive and come with a heavy number of US casualties. As a result, in recent years, Americans’ support for the US involvement in international affairs has declined. The grand strategy debate has further strengthened the appeal of people for the US to reduce its involvement in international affairs. The political parties have to be able to respond to the demands of voters if they want their support. The Obama administration has responded to the situation by adjusting and correcting its predecessors’ grand strategy. In the 2016 US presidential election, the foreign strategies and policies of the Democratic nominee Hillary Clinton and the Republican nominee Donald Trump were to some extent influenced by the grand strategy debate. They were both aware that the majority of American people are fed up with the government’s endless and reckless intervention in international affairs, and thus proposed their respective policies and ideas to cater to voters. 　　Second, the grand strategy debate has clarified the pros and cons of different options from historical, theoretical and practical perspectives, and has thus laid a solid knowledge base for US policy adjustments, enhanced the American people’s understanding and awareness of different options, and provided public support for policy adjustments.
　　The grand strategy debate has to reflect on the main problems in the United States’ foreign strategy. Only through the grand strategy debate, can these problems be clearly revealed and recognized by the people and a solution proposed and accepted.
　　Finally, despite the debate’s implications and influences, it is somewhat different from policymaking. Its greater role is to promote theoretical progress in the academic research on US foreign policy, and thus provide guidance to the development of specific policies.
　　As mentioned earlier, the grand strategy debate has had a significant impact on the Obama administration’s foreign strategy. When Obama came to power in 2009, the United States was strategically and militarily overexpanded and its relative strength in the international system was declining. This prompted the Obama administration to adjust the United States’ grand strategy. Under the banner of “change,” Obama believed that revitalization of the country was essential for any long-term grand strategy, since the US was suffering from excessive expansion, especially in the Middle East and the mistakes of the George W. Bush administration led the US to its lowest point with regards its international status. Based on these ideas, the Obama administration put more resources and attention on domestic issues, prioritizing economic growth, while its confidence and impetus to intervene in external affairs declined. It exercised more restraint in taking actions abroad, emphasizing “leading from behind,” multilateralism and “smart power;” it endeavored to mobilize allies and partners to take collective actions and share risks and responsibilities. This is the so-called “Obama Doctrine.”
　　During the Obama administration, the grand strategy of the United States contained three principles: First, to maintain the international order formed after the Cold War, and the leadership and primacy of the US that order has depended on. This was clearly stated in every important strategic document issued by the Obama administration. Second, to implement global leadership in a smarter, less costly and more prudent manner, especially when it came to the use of force, and the threshold for military intervention was raised to avoid involvement in a new war. Third, it pursued a strategy of “rebalancing to the Asia-Pacific,” because the Asia-Pacific is becoming the global center of security competition and economic growth in the 21st century, and China’s rise in particular poses the greatest long-term challenge to the US dominance of the global order. 　　In practice, the Obama administration implemented its policies in three core regions: In the Middle East, the Obama administration relied mainly on economic, diplomatic and intelligence operations rather than on military intervention, limiting its role to providing support and guidance, while focusing on more strategic issues such as missile defense and nuclear deterrence in the Persian Gulf region. In Europe, the United States called on NATO’s European member states to increase their defense spending and strengthen their staffing and capacity building. Letting them assume responsibility for European security and defense within the framework of NATO, the United States mainly played a strategic role, such as providing a nuclear umbrella, and the command and control system of intelligence, surveillance and reconnaissance (ISR). After the Ukraine crisis, the United States sent additional troops to Europe and put forward a military aid program called the European Reassurance Initiative, reaffirming US security commitments. In the Asia-Pacific region, the Obama administration launched its rebalancing strategy to strengthen ties with its allies and partner countries, expand its military presence, and optimize its military deployments in the region. It has used security issues such as the East and South China Seas and North Korea nuclear issues in order to strengthen the alliance system in East Asia and to deal with the impact of China’s rise on the regional landscape.
　　On the whole, under the Obama administration, the goal of the US grand strategy was to continue to maintain its dominant position in the international system, while emphasizing the need to achieve this goal in a more cost-effective and balanced manner, exercising some retrenchment and more restraint. This overall objective and the trend of retrenchment are expected to continue.
　　Conclusion
　　The US grand strategy debate attempts to point out the direction of US foreign policy adjustments in the transition period. In the context of the changing relative strength of the United States and the increasingly complex international environment, the United States’ strategic adjustments are aimed at maintaining the balance between means and objectives to preserve its global leadership through a less costly and more sustainable approach. Thus, retrenchment has become the main tone of its foreign policy adjustments. But retrenchment is not isolationism. It does not mean that the United States will abandon its global leadership role. Rather its retrenchment attempts to avoid excessive foreign intervention and excessive international commitments so the US can invest more resources and energy in responding to challenges that are more critical. 　　As Donald Trump officially becomes the 45th US president, the United States’ grand strategy will see more changes. From his campaign speeches, Trump’s foreign strategy and policy preferences reflect certain isolationist tendencies. He declared that the United States should reduce its interventions in international affairs, and should not act as the world’s police. He has advocated reconstruction of the relationship between the United States and its allies, demanding its allies bear some of the cost of US troops stationed overseas. His attitude is extreme on issues including immigration, globalization, free trade and many other issues. He shows little interest in the promotion of democratic values and human rights. But with regard to geopolitics and military security, Trump has showed a certain degree of policy confusion. He believes that the current US defense expenditure as a proportion of GDP is at its lowest level since World War II, and thus it is necessary to increase military spending to restore the United States’ absolute superiority in military force, which contradicts the strategic objective of retrenchment. Usually, there is a gap between election speeches and actual policies, and Trump’s foreign policy will be influenced by establishment Republicans. They have criticized the Obama administration’s foreign policy for being too weak, and believe that it has resulted in the decline of the United States’ international influence and the increasing external threats to US interests. It can be expected that Trump’s foreign policy is likely to lead to a new round of intense debate on the United States’ grand strategy.
　　Coping with the rise of China has been an important reason for the adjustments in the United States’ strategy in recent years. This will inevitably have an impact on China’s peaceful development and China-US relations. After Trump assumes office, the United States’ policy toward China may be adjusted. However, given the high degree of integration of the two countries’ interests and their interdependence in global affairs, maintaining the overall stability of bilateral relations will remain the United States’ best strategic choice. Faced with the adjustments in United States’ grand strategy, China should, on the one hand, maintain its strategic strength, continue to deepen its domestic reform and development, and take the initiative to shape the surrounding and international strategic environment. On the other hand, China should continue to promote the building of a new China-US relationship, use existing mechanisms to deepen cooperation in various fields, and expand the common interests of the two countries to promote the long-term, healthy and stable development of China-US relations.