Operation Barborossa, the German invasion of the Soviet Union, broke the German–Soviet Non-aggression Pact on 22 June 1941. The pact had been signed on 23 August 1939, and it had resulted in the Soviet Union and Germany dividing Poland between them. The pact also included a secret protocol, in which the Soviet Union agreed not to interfere with German plans for expansion in Eastern Europe.
However, Stalin began to suspect that Germany was planning to attack the Soviet Union, and he began to prepare for war. In May 1941, he ordered the Red Army to be placed on high alert. Hitler decided to pre-empt any Soviet attack and ordered the Wehrmacht to invade the Soviet Union on 22 June 1941.
The German invasion was a complete surprise to the Soviet Union, and it quickly overran large areas of Soviet territory. The Red Army was not able to stop the German advance until the Battle of Stalingrad in 1943. The German invasion of the Soviet Union was the turning point of World War II, and it led to the eventual defeat of Germany in 1945.