On June 22, the Soviet offensive on the Eastern Front, codenamed "Operation Bagration", almost destroyed the German Army Group Centre. Soon after, the Germans were forced to retreat and defend Ukraine and Poland. Arriving Soviet troops caused uprisings against the German government in Eastern European countries, but these failed to succeed unless helped by the Soviets. Another Soviet offensive forced Romania and Bulgaria to join the Allies. Communist Serbs partisans under Josip Broz Tito retook Belgrade with some help from Bulgaria and the Soviet Union. By early 1945, the Soviets attacked many German-occupied countries: Greece, Albania, Yugoslavia, and Hungary. Finland switched to the side of the Soviets and Allies.