When Germany invaded the USSR in June 1941 (Operation BARBAROSSA), the underground Bulgarian Communist Party launched a guerrilla movement, which was repressed severely by the government. After BARBAROSSA failed to defeat the USSR, and the US joined the Allies, it seemed that the Axis might lose the war. In August 1942, the Communist Party, the Zveno movement, and some other groups formed the Fatherland Front to resist the pro-German government. Partisan detachments were particularly active in the mountain areas of western and southern Bulgaria.