WWI

Have we forgotten that we shipped substantial material aid? The reality was that had the US not gotten involved the Germans would not have surrendered. Their population was being depleted just as badly as the Brits and French, with one difference. The russian revolution occured and the Germans were then in a position to focus on one front. Without the US, I don't think the continent would have survived. Britain maybe depending on whether or not they would be able to continue their success at Jutland.


WWII

Again, had the US not shipped goods, the Brits would have starved. The Wolfpack had in fact nearly accomplished this. Also, being the 'arsenal of democracy', I doubt the Russians would have faired so well. Had it not been for the US, Britain would have been starving and the Russians would have been defeated IMHO. To give you an idea of how much US industry contributed, after the war the US accounted for 50% of world GDP.

If it were a battle of Russia vs. the Germans, the Germans would have won. Without the US there would not have been a second front.

When you ask which wars we have won by ourselves is kinda misleading as most conflicts (including British) had allies and/or mercenaries.