That's my one issue too. Having played 3 times as the Entente on Hard mode against the AI, all three games have ended by the end of 1917. Usually Germany and France have such low morale by then that its just a race to see who dies out sooner.
This means that not only is US intervention unnecessary, there is no real point in fighting for Africa either as the Entente or frankly even worrying about AH or OE as once Germany's morale hits 0 the game is over.
So I attack with Russia, blockade with the UK and just try to hold in France, Serbia and Egypt. By Nov 1919, Germany is past saving and it just becomes a matter of when they will hit zero.
It feels to me that morale is about 1.5 to 2 times too fast.
I don't mean this as a shot at the game. Best SC2 yet, IMO. Just would like to see these countries fight longer in a pointless, unwinnable war.