I read the other day that if the Marines had not come in at Belleau Wood, the Germans would have taken Paris. Some conjecture that would have caused the allies to lose.
The French army by 1917 was pretty much a “spent force”. The 2nd Battle of the Aisne, was the last straw. It as not going to do any more suicidal assaults. This was when the French Army munity of 1917 occurred. Petain was the one that actually got the army calmed down. Basically it ws no more assaults, wait for the Americans.
The thing is if we had stayed out, the whole thing might have ended right there. The French would have been forced to negotiate with the Germans, I doubt seriously if he British would have been able to stop them Lurking in the back of everyone’s mind was the Russian Revolution. Many in the Allied governments were worried that they might be next. Of course it would have ended with Germany in a predominate European position and likely a Bolshevik Russia. However with reasonably strong Imperial Germany to oppose them.
This too would make for an interesting alternative history SciFi novel!