A detailed exploration of the most sweeping government border closures in human history during the Covid-19 pandemic and the implications for the future of global mobility. More people traveled internationally in 2019 than in any year in history. After COVID began its rapid spread throughout the world, though, international travel plummeted, and nations across the world hardened their borders. For the first time, governments took the same tools that have been used against less privileged migrants and asylum seekers and turned them on citizens from countries that had long enjoyed relatively unfettered travel--and sometimes on their own citizens. In When the World Closed Its Doors, Edward Alden and Laurie Trautman tell the story of how nearly every country in the world shut its borders to respond to an external threat and explain how this global shock to the system ended up transforming state border policies around the world. They detail the consequences of the COVID border restrictions--couples separated for years, children blocked from reuniting with their parents, container ship workers moving essential goods trapped at sea, pregnant citizens barred from returning home--and explain why governments used their harshest containment measures on those coming from outside. Throughout, Alden and Trautman focus on human stories to show the multiple impacts that states' increasing restrictiveness has had--economic, demographic, social, and political. And the fallout continues: governments left unchecked will continue to restrict borders with little regard to the collateral damage and disruption they cause. A sweeping overview of the re-bordering of the world, both during and after 2020, this synthetic, wide-angle view of a singular shock to the international systems of travel and migration highlights why citizens need better protections and governments more robust guardrails.