What was the most decisive event during the World War I?
I think that the most decisive event in World War I would be the American entry into the conflict. Prior to American entry, there had been a muddle of who was winning. Central Forces and Allied Forces would wage battle endlessly for what amounted to be two or three steps' worth of land. No side was clearly exerting its will. Like two prize fighters on the ropes, both sides were weakened by the sheer force and magnitude of the other.
With the American entry into the war, the Central Forces lacked the ability to carry on. The sheer reinforcement of military personnel was decisive in the Allies' favor. At the same time, the technical aspect of new and more ammunition, supplies, and rations overwhelmed the German forces. Suffering under the weight of attrition, lacking basic necessities like food, and weapons, the Germans felt the crushing power of American entry. Allied victory only became lucid after the American entry into the war. It was at this point in which the balance of the war tipped to the Allies' favor.