Germany pressured Yugoslavia to become part of the Axis. On March 25th the regent Prince Paul agreed. The action was unpopular in the country and in the army, which staged a revolt on March 27th.

Hitler was incensed by the decision and ordered Yugoslavia to be considered a hostile state. On April 6th the Germans began an invasion of Yugoslavia. The Germans used their air force first and attacked Belgrade and all the installations of the Yugoslav air force. The German army attacked on land from Bulgaria while by Hungarian army attacked from the North, and on the April 11th the Italian army joined in. The Yugoslav army quickly collapsed. The country was divided up between the various invading countries. In addition a number of puppet states such as the Independent State of Croatia were established.