When Donald Trump assumed the presidency in January 2017, the central tenets of U.S. foreign policy were already under scrutiny. Challenges came from within the United States, in the form of discontent with the unequal effects of globalization and the costs of foreign intervention, and from abroad, as rising powers contested post–Cold War U.S. dominance. Guided by an “America first” ethos, the Trump administration deviated from the United States’ traditional approach to alliances and international cooperation, favoring transactional relations and a renewed focus on geopolitical competition. It reversed U.S. policy in key areas, from abrogating the Iran nuclear deal to withdrawing from the Paris climate accord to recognizing Jerusalem as the capital of Israel. It escalated some conflicts, including a trade war with China, and struggled to extricate the country from others, as in the case of U.S. military operations in Afghanistan and Syria. And, following a final year in office consumed by the coronavirus pandemic, Trump will pass on to his successor a global health and economic crisis that has come to define his presidency. In this collection, we look at how Trump’s policies and the reordering of global politics over the past four years have contributed to a broader reassessment of the United States’ role in the world. That the Trump era marks a sharp divergence from the recent past is clear, but the consequences of the shift are less so. Grappling with this legacy will be among the next administration’s most significant foreign policy tests.