The US won the revolutionary war because they outlasted the British will to continue the war, and Britain lost because the US outlasted their will to continue the war.

And Vietnam is the same way, assuming you count the eventual conquest of the South two years after the US was done and gone as a US defeat.

The Allies (read: Russia, and Western Strategic Bombing) won, largely, because they employed a cohesive strategy and had the will and the material to carry it to its conclusion.

Whereas Germany lost, not because the Allies employed a cohesive strategy and had the will and material to carry it out, because its very basis as a society was fundamentally flawed. This led to inflexibility which magnified all other problems it had and blunders which its leaders made. Had it not had a faulty foundation, it would not have made the poor decisions in the first place and then would not have fallen into its position.

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And the American Civil War.

The North won because it was able to fashion a doctrine and logistical basis (men, material and the means to move them) which fit its task, because they were able to destroy the South's will to fight.

Whereas the South lost, not because of material disadvantages (no Southern army ever lost a battle because of inadequate supplies or a manpower differential able to negate the defensive power of the rifled musket), but because they employed poor strategic and tactical decisions. Had Lee not destroyed his army in the year and a bit between the Seven Days and Gettysburg, he wouldn't have ended up getting bottled up in siege. Had the Confederacy in general not ignored the Western theatre, the North would never have been able to stage the great raids which finally broke the Southern will and revealed their nationalism for the fraud it was.