Now, I'm not much of a history buff, but the US's participation in WWI wasn't as conclusive as it was in WWII. "what if" 's are always guesswork, but Germany was close to starvation in 1918 thanks partly to the British blockade. It doesn't seem inconceivable that the Allied powers would have won eventually regardless of American intervention, but obviously at much greater cost. In the case of the US actively waging war on the Allied forces, that would have been a completely different story. However, one must consider the context. Until 1917, America stayed neutral and tried to avoid participation in the war. But, because of continued German attacks on American merchant ships, they were practically forced to retaliate (
source). The US never became a formal member of the Allied Powers though, they were a
self-styled "Associated Power". So you see, it's less that the US choose to ally with one side or the other, it's more that they retaliated after being unwillingly dragged into the war that spanned the globe.
P.S.
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We could have also helped Germany by sending it supplies exclusively.
As mentioned in one of the sources above, the US traded with both parties (for it's own profit), and both parties tried to prevent this for the other by creating a blockade. The British were more tactful and succesful about this though, avoiding to spill too much American blood lest it incurred the US's wrath. This probably ended up being one of the major deciding factors of the war.