The repatriation of the Cossacks or betrayal of the Cossacks occurred when Cossacks, ethnic Russians and Ukrainians who were opposed to the Soviet Union and fought for Nazi Germany, were handed over by British and American forces to the Soviet Union after the conclusion of World War II. Towards the end of the European theatre of World War II, many Cossacks forces with civilians in tow retreated to Western Europe. Their goal was to avoid capture and imprisonment by the Red Army for treason, and hoped for a better outcome by surrendering to the Western Allies, such as to the British and Americans. However, after being taken prisoner by the Allies, they were packed into small trains. Unbeknownst to them, they were sent east to Soviet territories. Many men, women and children were subsequently sent to the Gulag prison camps, where some were brutally worked to death. The repatriations were agreed upon at the Yalta Conference; Soviet leader Joseph Stalin claimed that the prisoners were Soviet citizens as of 1939, although there were many of them that had left the country before or soon after the end of the Russian Civil War or had been born abroad, hence never holding Soviet citizenship.

The Yalta Conference, held 4–11 February 1945, was the World War II meeting of the heads of government of the United States, the United Kingdom and the Soviet Union to discuss the postwar reorganization of Germany and Europe. The three states were represented by President Franklin D. Roosevelt, Prime Minister Winston Churchill, and General Secretary Joseph Stalin. The conference was held near Yalta in Crimea, Soviet Union, within the Livadia, Yusupov, and Vorontsov palaces.