Before the US intervened, the Germans were deadlocked with the Russians, French, and British. The French would have folded up even faster than they did in 1871 if the British hadn't bailed them out in 1914.
In all four countries, the war chewed up most of the current generation of young men. If the US had not entered the war--and in my opinion, from the US perspective, there was no compelling reason (other than the yellow journalism of the British press, which spent four years accusing the "Huns" of everything short of eating babies) to get involved, as in 1914 the US had friendly relations with all the powers involved and none of them declared war on the US--it would probably have gone on for decades and sent every industrialized society in Europe back to the Stone Age. The sudden entry of the US in the war, with vastly greater reserves of manpower, industrial capacity, and natural resources than all of Europe put together at the time, tipped the balance in favor of the Allies at the time that everyone involved was near the breaking point.
Meh. In 1914 there were too many mediocrities and out-and-out fools in positions of power in Europe, due (in my opinion) in large part to the corrupting influence of hereditary aristocracies. There was no good reason for war to start in 1914, other than maybe the jingoistic French desire to avenge the drubbing the Germans handed them in 1871--that, and the Kaiser's generals rubbing their hands gleefully, hoping for a rematch where they'd be able to take Paris this time.