I'm speaking in regards the the War in Europe during WW2. Definitely the USSR, however this could be looked at differently. USSR initially had a part to play in beginning the Second World War, as Stalin and Hitler had formed a pact of non-aggression between the two nations. They eventually coordinated the invasion in Poland, where the soviets and Germans invaded from the east and west of Poland (Germany from the west, Russia from the east). However , tensions between Hitler and Stalin grew as time went on (always the way with left and right- cannot be cohesive). The increased tensions eventually lead to hitlers betrayal of the pact of non-aggression and the beginning of the war between the soviets and the Germany's. So Stalins original plan of non aggression with Hitler backfired and that's the only reason the USSR entered the war (to defend themselves). On the other hand the Americans had no obligation to go to war in Europe. In fact they only joined once things were looking very dismal in 1944, where the whole of mainland Europe was occupied territory by the nazis. Then the Americans show up, and spearhead the allied invasion of Europe. During this time the Americans were already fighting in the Pacific, which leads me to believe that there was more selfish reasons for the US assisting in the European war. I understand that it was two different areas of the US armed forces fighting in two difference theatres of war, but it just doesn't make sense why America wouldn't concentrate all there resources on the immediate Japanese threat. To sum up I believe Americans had a greater part to play for WW2 as a whole, because they were fighting in two theatres of war, whilst the USSR originally causes the war in Europe then ended up needing to defend themselves from hitler once he went back on the pact.