Soviet army formed by Trotsky as Commissar for War (1918–25) to save the Bolshevik revolution during the Russian Civil War. For trained officers, Trotsky had to rely on former officers of the Imperial Army. After Hitler's invasion of the Soviet Union (1941) the Red Army became the largest in the world – reaching five million by 1945. Precise figures remain unknown, but Red Army casualties in World War II have been estimated as high as seven million men. The name fell into disuse shortly after World War II and was replaced by that of Soviet Armed Forces, which in turn adopted the titles of the independent republics after the breakup of the Soviet Union in 1991.