According to my information on History, United States was the main player of WW1, so I would say it was the dictator of the WW1. He was the main player in the sense that after the entry of US in the war, it took a final shape. It declared war on Germany and eventually the Allied forces were victorious.
The U.S entered the war in the final year of the conflict. Their participation was key in ending the war when it did yes, I don't want to start an argument but I would hardly say they were the main player.