Stalin joined forces with the Allies in World War 2 primarily to defeat Nazi Germany. The German invasion of the Soviet Union, known as Operation Barbarossa, began on June 22, 1941, and posed a significant threat to the Soviet Union's existence.
Initially, Stalin sought to maintain neutrality in the war and had signed the Molotov-Ribbentrop Pact with Nazi Germany in 1939. However, the invasion of the Soviet Union by Germany led to a dramatic change in Soviet foreign policy.
After the German invasion, the Soviet Union desperately needed military and economic aid from the Western Allies, namely the United States and Great Britain. This led to the formation of the Allied coalition, with the common goal of defeating Nazi Germany and its Axis allies.
The Allies provided the Soviet Union with vital supplies, including food, ammunition, vehicles, and aircraft, under programs such as Lend-Lease. The United States also provided significant financial support to the Soviet war effort.
Despite ideological differences, the Allies recognized that defeating Germany was of paramount importance. The alliance between the Soviet Union and the Western Allies was primarily a marriage of convenience, with both sides seeking to preserve their own interests in the face of the common enemy.
Ultimately, the Allied forces, including the Soviet Union, emerged victorious in World War 2, leading to the defeat of Nazi Germany and the end of the war in 1945.