Britain on the first occasion. We didn't need to get involved in the First world war and we still saved their asses. If the British Expeditionary Force hadn't slowed the Germans down the French would've failed to stop the Germans and Germany would have won. However, the joining of the Americans saved countless British and French lives and shortened the war considerably. They do owe the Americans a debt there, but it is important to remember that the Americans entered the war for their own reasons, not because they wanted to help Britain or France particularly. If I remember, they entered the war because Germany was sinking their shipping. If you ask me, the Americans would have been wiser to stay out and make a fortune off the loans to the various parties. I can't really say that either of the sides deserved to win, Frances president turned out to be a real bastard towards Germany and America could have claimed the moral high ground and stayed the hell out of a needless fight, like Switzerland. You can partially blame Clemenceau for the second world war. Germany should have been left alone, not bombarded with reparations. Contrary to (not so now) popular belief, Germany did not start the war, you can blame the Russians for that when they attacked Serbia instead of leaving the matter to the Austrians. The Russians did not have an alliance with Serbia, they attacked just to protect their sphere of influence. The Germans, on the other hand, did have an alliance with Austria, and mobilised accordingly. France had an alliance with Russia, and Germany knew this, so they naturally struck at the smaller country first in the hopes of knocking it out the war quickly. Of course, this back-fired.
On the second, I'd say both the common-wealth and America. Not only did commonwealth forces hold the line both in Europe and in North Africa, reducing the Germans western offensives to the point of mop-up, but they also did a lot to help reduce the air-power of Germany. Commonwealth efforts paved the way for the liberation of Europe while everyone elses efforts bar Russia's were failing... but admittedly that was just self defence, though Churchill clearly had ideas for retaking Europe. The common-wealth would also never have managed to defeat nazism on their own, though I still think that the Germans were on their way out and could've been defeated without America. The best they could've hoped for was to drive the Nazi's out of France. Russia would've gained more ground, probably taking the whole of Germany and even part of France. So ultimately, I'd say America takes the credit of ultimately being Frances liberator, while the common-wealth and Russia takes the majority of the credit for stemming the tide and halting Germany's advance. I also have a much more positive view of the Americans involvement in the second world war. They genuinely seem to have made that rare thing in world events, a selfless decision to help us and to stop Germany from taking over Europe, and Europe owes them a great deal for that. That does not mean, however, that Europe has to follow everything the USA says. The best friends will tell you when you are wrong.
I have an incredibly low opinion of Russia's, or should I say, Stalins involvement in the war. They just seized as much land as they could, as violently as possible. That is more a comment on Russia's leadership at that time than on Russia itself though...
Of course, they were all a bunch of cynical, ruthless bastards in the end. Allied forces bombed German cities into rubble, while the less said about the Russian atrocities the better. The allied forces in the west were at least more merciful than their German and Russian counterparts (or maybe just more disciplined when it came to treating civilians and captives), but it is important to remember that all parties in that war were incredibly ruthless and thought little of killing civilians on a massive scale. The horror after that is the reason we have the EU, no one wants to go through that again. No one.
You may think from this post I'm cynical. I'm not, I just seemed to have picked up a depressingly bleak view of history and why things happened. Humans are nice. Nations are bastards, particularly towards their neighbours. If we just abolished armies and borders and had a really effective police force and democratic government, the world would be a much safer place... until aliens invade or a dictator takes over that is...
(whoops, sorry, misunderstood your post - I thought you meant more manpower than other allies, I realise you now meant more manpower than WW1 - which is correct - but the other nations put more manpower into WW2 than US did as it happens - I shall elaborate as I can't be bothered to delete what I had already typed)
