The United States joined the war and Russia dropped out. This helped swing the war to the Allies side and also made it more of an ideological war.
The entry of the US into the war helped swing things towards the Allies because it brought in huge numbers of fresh troops and all their supplies. This broke the stalemate that had arisen.
The exit of the Russians affected the war to some degree because it made it into a war of democracies against monarchies. This helped make the US more enthusiastic about participating in the war.
The US joined the war in 1917, which changed everything. Massive numbers of American troops and a brand new, freshly equipped fighting force would tip the balance on the Western Front in favor of the Allies and force the Germans to eventually seek a cease fire.
At almost the same time, the Russians were forced to drop out of the war due to the Russian Revolution and their inability to fight both the German Army and the communist rebellion, or more accurately, the communists ability to fight a civil war with the Czarist nationalists.
This freed a large portion of the German Army and allowed them to focus the final offensive of the war only on the Western Front. It very nearly gained them victory in the whole shooting match.