Matt Quinn said:
This is WWI, right?
If there're no US troops to hold the trenches and no promise of eventual US support, the Germans would probably have mauled the Brits and French. The Allied armies (esp. the French) were unbelievably demoralized due to their commanders' penchants for suicidal offensives.
By the time of the spring offensive there were very few US troops at the front in France, and they played no significant role in stopping the offensive. The offensive was mainly stopped by the British 5th Army, which, although it took extremely big losses, in the end kept the front. By end of April most losses had been replaced -quite remarkable if an collapsing army.
In short it seemed like the British Army had fully overcome it's morale problems of 1917. Some claim that the French hadn't, but I've only heard that from Anglo-Saxon historians, and they had (have) a bad habbit of only superficial knowledge about allies (and enemies). But if it in any way is true that the French were still weakened in 1918, then placing the main attack in the French sector instead of the British perhaps would have had the Entente front collpase.
The later very big US contingent did however play a major role in the Entente autumn offensive and the German collapse in November. Without US participation in the autumn offensive the Germans probably still wouldn't be able to win the war (they were a spent ball after the spring offensive), but it probably wouldn't have been possible to impose a Versailles Treaty on them.
Regards
Steffen Redbeard