On 23 August 1939, after unsuccessful efforts to form an anti-fascist alliance with Western powers, the Soviets signed the non-aggression agreement with Nazi Germany. After the start of World War II, the formally neutral Soviets invaded and annexed territories of several Eastern European states, including eastern Poland and the Baltic states. In June 1941 the Germans invaded, opening the largest and bloodiest theater of war in history. Soviet war casualties accounted for the majority of Allied casualties of the conflict in the process of acquiring the upper hand over Axis forces at intense battles such as Stalingrad. Soviet forces eventually captured Berlin and won World War II in Europe on 9 May 1945. The territory overtaken by the Red Army became satellite states of the Eastern Bloc. The Cold War emerged in 1947 as a result of a post-war Soviet dominance in Eastern Europe, where the Eastern Bloc confronted the Western Bloc that united in the North Atlantic Treaty Organization in 1949.