In my opinion, It would probably be the USSR in first because Germany didn't really start to waver until Russia did what Russia did best and gave them a whole bunch of icy cold winter, pushing them back. One reason the allies started the western front because they didn't want the Soviets to control all of Germany, so they figured they would control some of it before Stalin did. The UK was like the last man standing and in reality, they didn't do a whole lot, but just their existence was enough to help. Germany was actually very scared of Britain and would never try to invade it, which is why they only bombed it. Britain was doing what Britain does best: Rule the waves. Germany didn't have a very good navy, and I believe their best shot at gaining control over the channel came in the form of the Bismarck. But that didn't work, and it reaffirmed to the Germans that if they tried to cross the channel, even with aerial superiority, they would just be decimated by the British. This is also reinforced by the battle of Dunkirk, where family fishing boats were able to make it across the channel safely, with the only threat being the german Bombers. The USA didn't actually do much against Germany, because by the time they joined Germany was pretty much-taken care of. Where the USA shines is in the East Asian Theatre against Japan. As far as I know, they were either the sole reason or heavily helped with the taking down of Japan. They were going to Nuke Germany, but since Germany was already taken care of, they used them on Japan instead.
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In my opinion, It would probably be the USSR in first because Germany didn't really start to waver until Russia did what Russia did best and gave them a whole bunch of icy cold winter, pushing them back. One reason the allies started the western front because they didn't want the Soviets to control all of Germany, so they figured they would control some of it before Stalin did. The UK was like the last man standing and in reality, they didn't do a whole lot, but just their existence was enough to help. Germany was actually very scared of Britain and would never try to invade it, which is why they only bombed it. Britain was doing what Britain does best: Rule the waves. Germany didn't have a very good navy, and I believe their best shot at gaining control over the channel came in the form of the Bismarck. But that didn't work, and it reaffirmed to the Germans that if they tried to cross the channel, even with aerial superiority, they would just be decimated by the British. This is also reinforced by the battle of Dunkirk, where family fishing boats were able to make it across the channel safely, with the only threat being the german Bombers. The USA didn't actually do much against Germany, because by the time they joined Germany was pretty much-taken care of. Where the USA shines is in the East Asian Theatre against Japan. As far as I know, they were either the sole reason or heavily helped with the taking down of Japan. They were going to Nuke Germany, but since Germany was already taken care of, they used them on Japan instead.
So My ranking would go:
1.) USSR
2.) UK
3.) USA