The German attempt to get Mexico to attack the US was a mess for them, and perhaps led to our declaration of war as much as the sub thing.
Had neither happened and Germany prevailed in the war, history would be vastly different.
The AEF was fairly small in numbers relatively speaking, but provided some critical "backbone" when the French army was seeming to give up.
The French already had issues with mutinies in earlier years. The Germans were limited in their 1918 advance by logistics, and their hungry troops often would sit down in a town and just eat for a period of time.