now there has been a lot of people saying that the Americans shouldn't be in the game because they joined the war later than anyone else, but what you must realize is it's not about how long the Americans were in the war it was the fact they pretty much ended WW1.
now if you are not understanding, the American forces in WW1 had some very smart people leading assaults. not only did the American forces have smart people, but they also had the energy advantage over any of the allies. this isn't mean to stir up anything it's just fact the Americans weren't worn out because they weren't in the war until the end.
you may also not know, but the Argonne Offensive was very influential to the end of the war. the idea of pushing the Argonne forest was to break the german line and the plan did that pretty well.
so although the Americans were not in the war as long as the other allies, they did have a huge part in ending WW1. probably the biggest.