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.