WWI - US intervened and insured victory for the allies then went home and left a festering mess (Treaty of Versailles) which eventually became WWII.

WWII - the US intervened way to late, but when they did, stayed around to clean up the mess (Marshall plan) leading eventually to a prosperous, united, peaceful Europe and Japan.

If you think of it as one drawn out conflict, you can certainly say that The US did the right thing in the end.