I've been bad about keeping up with historical posts, but this is worth noting: 100 years ago today the US declared war on Germany in World War I. (The Great War, back in the day.) It was an important act - the US pretty clearly tipped the balance in favor of the Allies, at a time that the Russian Revolution was starting to look like it would tip the balance in favor of Germany. The effects took another year to show up on the battlefield - 1917 would be another miserable year for all concerned, and leave France pretty well neutralized as a force, and the UK not far behind - American troops turning up in 1918 would turn it back. Though just the presence of the Americans in the war, fully committed to the cause of the Allies, and thus to their economic and naval warfare against Germany would have almost as much impact as those fresh soldiers would.
The Germans - well, they were in a hard place in the first world war - submarine warfare brought the US in - but without it, they were going to be slowly starved into submission (while their enemies were not.) Submarines were their best chance to win the war - but they turned the world against them, so they had no chance. One of those things. Wars are never entirely military. Politics and diplomacy never go away - economics never goes away.
As for the US - we didn't get much out of our involvement. Woodrow Wilson dreamed of using the war to bring about a peace that would stabilize the world - that didn't work out (to put it mildly.) There were widespread crack downs on civil liberties in the US - it led to prohibition - it probably helped spread the influenza epidemic that killed more people than the war - and killed 100,000 odd Americans outright. Whether this saved anyone, improved the situation in Europe in any way, is anyone's guess. But off we went,a nd took our place in one of the worst disaster ever to befall humanity.