Why did the Soviet Union become an Allied power in 1941?

The Soviet Union became an Allied power in 1941 because Nazi Germany launched Operation Barbarossa, a surprise invasion of the Soviet Union, on June 22, 1941.123

Prior to this attack, the two nations had been bound by the German-Soviet Nonaggression Pact of 1939, which secured Germany's eastern flank and divided eastern Europe into spheres of influence. Although Soviet leader Joseph Stalin had hoped to avoid conflict with Germany, Adolf Hitler’s profound anti-Bolshevism and his desire to conquer the Soviet Union led him to break the pact.124

Following the invasion, the Soviet Union and the United Kingdom signed an agreement on July 12, 1941, formalizing their partnership against the Axis powers. Unlike other nations that had been quickly overrun by Germany, the Soviet Union was able to withstand the initial assault and became a critical member of the Allied coalition, eventually receiving support through the American lend-lease program.3