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What it says on the tin in 2 or fewer pods.
Winning doesn't have to mean the Tricolour flying over Berlin (didn't even happen in OTL) but can just be a negotiated peace that obviously favours the Entente. The U.S. can join whenever you want but they should do some fighting (to rule out things like the U.S. symbolically joining the day before an armistice is signed and not fighting a single battle.