What do you believe was the turning point of the outcome of the First World War? Why?

In my opinion, the turning point in this war was the entry
of the United States.  I hope I am not saying that just because I'm American.  My
reasoning is that the war had been stuck in a total stalemate from 1914 all the way to
the time that the US entered.  After that, the war ended rather
quickly.

The US entry was important because the US had so
many men that they could throw into the battle. When you have two sides in a war who are
so evenly that they can't move each other for years, and then you throw a huge new army
in on one side, of course it's going to make the decisive
difference.

So the US totally changed the balance of forces
available when they entered the war.  That is why I think their entry was the most
important factor in the outcome of the war.