The successful 70-year campaign to convince people the USA and not the USSR beat Hitler

Assessing the "biggest contributor to victory" in a rigorous way is exceptionally difficult. They tend to devolve into comparisons of counterfactuals, and the truth is that nobody has any strong idea how the war would have turned out absent US involvement, or if the German-Soviet non-aggression pact had held, etc. But the case is pretty strong that the Soviet Union's successful resistance of Nazi invasion and subsequent reclamation of Eastern Europe was the most important of many crucial factors in defeating Germany. As historian Richard Overy explains in his book Why the Allies Won:

If the defeat of the German army was the central strategic task, the main theatre for it was the conflict on the eastern front. The German army was first weakened there, and then driven back, before the main weight of Allied ground and air forces was brought to bear in 1944. Over four hundred German and Soviet divisions fought along a front of more than 1,000 miles. Soviet forces destroyed or disabled an estimated 607 Axis divisions between 1941 and 1945. The scale and geographical extent of the eastern front dwarfed all earlier warfare. Losses on both sides far exceeded losses anywhere else in the military contest. The war in the east was fought with a ferocity almost unknown on the western fronts. The battles at Stalingrad and Kursk, which broke the back of the German army, drew from the soldiers of both sides the last ounces of physical and moral energy.



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