More than 3.5 million people have fled Ukraine since Russia invaded on Feb. 24, the fastest-moving refugee crisis in Europe since the end of World War II. The massive displacement of millions of people threatens not only a humanitarian disaster but an ongoing challenge for European nations and the United States. Elisa Massimino is a Senior Fellow at the Center for American Progress and Executive Director of the Human Rights Institute at Georgetown Law. She led a team that recently issued a report titled “What the European Union and United States Need to Do to Address the Migration Crisis in Ukraine.”