The Soviet Union's entry into World War II was a direct consequence of the non-aggression pact it had signed with Germany in August 1939. The pact, known as the Molotov-Ribbentrop Pact, divided Poland and the Baltic States between the two countries, and also included a secret protocol that outlined the Soviet Union's territorial ambitions in Eastern Europe.
Invasion of Poland:
On September 1, 1939, Germany invaded Poland, prompting France and Great Britain to declare war on Germany. Two weeks later, in accordance with the Molotov-Ribbentrop Pact, the Soviet Union invaded Poland from the east. The Soviet Union's actions in Poland were condemned by the Western Allies, and the Soviet Union was expelled from the League of Nations.
Winter War with Finland:
In November 1939, the Soviet Union invaded Finland in an attempt to gain control of the Karelian Isthmus, a narrow strip of land between Finland and the Soviet Union. The Finnish army, despite being outnumbered, fought a fierce resistance, and the Soviet Union was forced to accept a peace treaty in March 1940 that gave the Soviet Union some of the territory it had sought in exchange for ending the war.
German Invasion of Soviet Union:
The tension between Germany and the Soviet Union continued to escalate, despite their non-aggression pact. In June 1941, Germany launched Operation Barbarossa, a surprise attack on the Soviet Union. The Soviet Union suffered heavy losses in the early months of the war and were forced into retreat, but the Red Army eventually managed to stop the German advance and turn the tide of the war. With over 27 million casualties, the war against the Russian front was the single largest front of World War 2.
Overall, the Soviet Union entered World War 2 as a result of its alliance with Germany through the Molotov-Ribbentrop Pact, which divided Poland and outlined Soviet territorial ambitions. It aimed to secure territorial gains and maintain its influence in Eastern Europe. The German invasion of the Soviet Union in 1941 marked a significant turning point in the war and led to a prolonged conflict between the two countries that dramatically changed the course of the war.