When an hegemonic power, such as the US, terminates its support for international institutions it has helped to create, this poses a severe challenge to them. This book explains why some institutions are resilient while others decay after hegemonic withdrawal. It introduces the Leadership Transition Theory (LTT) and argues that the agency of alternative leaders is key to institutional resilience. The LTT posits that international institutions are more likely to be resilient after hegemonic withdrawal when actors are willing and able, due to their control over soft and hard power resources, to exercise alternative leadership. To assess the LTT empirically, this book focuses on the “American century” and employs a multi-method approach. It introduces an original, comprehensive dataset about the withdrawal of US support from international organizations and agreements in the period of 1945-2020 and their subsequent resilience or decay. The book not only assesses the LTT statistically but also provides four detailed case studies – the World Trade Organization (WTO), the “Iran Deal”, the the United Nations Population Fund (UNFPA), and the UN Relief and Works Agency for Palestine Refugees in the Near East (UNRWA) – and additional examples leveraging more than 60 interviews and background conversations with senior officials from various abandoned institutions’ administrations and their membership. The book demonstrates that US withdrawal is a recurring phenomenon not limited to the Trump Administration and that abandoned institutions are resilient when other Western governments and IO bureaucracies provided alternative leadership. The results of this book therefore come with important implications for the proponents of the international order to ensure the durability of international institutions against Trump 2.0 and beyond.