During U.S. President Donald Trump’s second term, his administration has been signaling the possibility of U.S. retrenchment from Europe (and possibly other regions) to promote allied burden-sharing. Evaluating whether such a change serves U.S. interests involves considering a variety of effects, including the behaviors of U.S. rivals, regional stability, and U.S. defense budgets. RAND researchers analyze one aspect of this broader calculation: how retrenchment affects the behavior of U.S. allies. To do so, they consider lessons from cases of U.S. retrenchment from West Germany, South Korea, Japan, and Taiwan in the 1960s and 1970s. The United States has long viewed itself as a global security leader, so it has not entirely retrenched from key regions. Still, the United States has engaged in limited retrenchment, which means reducing (but not eliminating) U.S. military involvement in a country or region by withdrawing U.S. forces, downgrading U.S. commitments to allies or partners, or providing less military assistance. Contemporary proponents of limited retrenchment argue that it would incentivize U.S. allies to do more for their own defense, moderate their ambitions, and act in ways that promote regional stability. However, these arguments run counter to the central tenets of post–Cold War U.S. grand strategy. To evaluate these competing beliefs, RAND researchers assessed how these historical U.S. force drawdowns and accompanying policies affected the perceptions and behaviors of these four U.S. allies, including effects on nuclear proliferation.

At least since the U.S. invasion of Iraq in 2003, a subset of realists, conservatives, and progressives have called for greater restraint in U.S. grand strategy. Given that restraint is a key position in the grand strategy debate, this article explores the variations within restraint's big tent in greater detail. Drawing on writings by and interviews with advocates of restraint, we identify the underlying beliefs that have led these disparate groups to converge on restraint. We find that there are competing visions of restraint. Realists want to prevent the emergence of regional hegemons without provoking great power war; conservatives seek to preserve what they identify as the American way of life; and progressives are motivated by the desire to combat inequality and injustice at home and abroad. Even within each group, there are a range of underlying beliefs and foreign policy preferences, and more divisions have emerged in response to the Russian invasion of Ukraine and China's rise. We explore how these groups' positions may evolve in the future as the threat environment continues to change and new policy questions emerge.

Debates in the United States about policy toward Taiwan tend to focus on the choice between strategic ambiguity and strategic clarity and how these options affect China's calculus on invading the island. The authors expand the discussion by considering how Japan, the Republic of Korea (ROK), and the Philippines would react to a range of policies the United States may choose to signal either an increase or a decrease in support to Taiwan. To do so, the authors reviewed each ally's historical relationship with Taiwan and the contemporary relations with the United States, China, and Taiwan. They also interviewed policymakers and experts in Japan, the ROK, and the Philippines about their views on hypothetical U.S. policy changes, including potential changes in U.S. diplomatic, intelligence, military, and economic relations with Taiwan. The authors find that Japan favors increases in many forms of U.S. diplomatic and military support to Taiwan and would adopt similar policies up to a point. The ROK and the Philippines favor the status quo and are less likely to increase support to Taiwan. All three allies oppose reductions in U.S. support to Taiwan, both because they believe it might lead to instability in the Taiwan Strait and because they would interpret reduced U.S. support to Taiwan as a signal of waning U.S. commitment to their own security. Such concerns would likely lead Japan and the ROK to try to draw the United States closer and increase defense spending. The Philippines' response would likely depend more on who is in power and China's recent behavior.