Victory Over Germany and Italy:
15% UK + the British Empire
Victory Over Japan:
5% USSR (1939 USSR soundly defeats an attempted Japanese invasion)
10% mixture of UK, Australia, India and a long list of other contributors
You have to be careful with the thought process that the USSR could have won it completely on their own. For one, the Soviet Union was saved from a total disaster by Hitler's meddling in 1941. It could have gone very badly for the Soviets right there.
The USA sent craploads of material to the USSR. It is my belief that even though this constitutes only 10-15% of the material the Soviet Union used, it was enough of an edge to help them manage to turn the tide. Russia was a difficult place to reach, but the USA and UK did the best they could to help out the USSR any way they could. Because the UK was in it as of the beginning of Barbarossa, there was a lot of production focussed on the U-boat Wolfpacks that could have been focussed on the USSR. The bombing campaign by the USA and the UK was more useful for diverting German production and attention than actually destroying German industry, but that diversion was that much more focus that the USSR didn't have to deal with. Maintaining a strong defense force against amphibious invasion throughout the war also diverted manpower and industry that could have been focussed solely on the USSR.
Give credit where credit is due -- the USSR turned the tide on the Eastern Front mostly going it alone. The USA and UK bombed a lot and most of that bombing campaign was sold to their citizens as being a helluva lot more effective than it actually was. The USSR pushed the Germans back with a terrible price in blood, resources and devastation to their nation.
"It is well that war is so terrible, else we should grow too fond of it."
- General Robert E. Lee
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