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.