During World War II, Russia (the Soviet Union at the time) played a decisive role in the European arena. The Soviet Union was a key member of the Allies and was instrumental in the defeat of Nazi Germany, particularly following the Battle of Stalingrad and the subsequent offensives that pushed German forces back into Germany.
While there were initial non-aggression pacts and some cooperation between Nazi Germany and the Soviet Union (such as the Molotov-Ribbentrop Pact), the overall contribution of the Soviet Union to the war effort against the Axis powers was significant and critical in shaping the outcome of the war in Europe.