Economically and militarily, the U.S. was the deciding factor in World War One. The war was a virtual stalemate prior to U.S. intervention, although the British worked to bring the U.S. into the war, and the Germans worked to prevent it.
When the U.S. entered the war, it was able to amass an Army of four million men. Similarly it was the world's largest economic power, and was soon prepared to totally mobilize its resources to support the war effort. When it appeared inevitable that the U.S. would enter the war, the British worked to prolong the conflict while the Germans tried to end it quickly. In fact, when told by Gen. Ludendorff that the U.S. had declared war on Germany, Field Marshall von Hindenberg wired him back simply, "make peace, you idiot."
The U.S. entered the war with fresh men and supplies when both sides were exhausted. It is not coincidental that the war ended just over one year after the U.S. intervened. A strong argument can be made that U.S. entry into the war not only brought it to a sooner conclusion, but probably saved the lives of millions who might have died had the war been prolonged.