I think the war would have to end before the U.S. deployed troops to France at the latest. I read a book a couple years ago that traced the spread. It may have even spread to both sides of the front before then. So even if the war ended sooner the return of troops to their home countries would have allowed the spread. Maybe the lack of censorship would have allowed the spread of information to have caused quarentines and other measures to be adopted but I don't think it would have really stopped it.