Here are a few events that we can say were important in allowing the Allies to win World War I.
September 1914. The First Battle of the Marne stops the German advance through France. If the French and British had not won this battle, Germany might have managed to force France to surrender and the war might have been over very quickly. Another battle that had a similar effect was the First Battle of Ypres in September and October of 1914).
January 1917. The Zimmermann Telegram is released in the United States. This helps makes Americans angry at Germany. That helped the Allies because it helped cause the US to enter the war.
February 1917. Germany resumes unrestricted submarine warfare. Germany had stopped sinking ships without warning because it did not want to antagonize the US. However, it now decided that it had to resume unrestricted use of its submarines so it could try to end the war quickly. This helped the Allies because it helped bring the United States into the war.
April 1917. The United States enters WWI. This did not have an immediate effect, but it would be a major factor because it eventually gave the Allies more soldiers and supplies to use in the war.
April 1918. The Allies stop a German offensive on the Western front. The Germans launched the offensive to try to end the war before the American soldiers really joined the fighting. Battles include the Battle of Cantigny.
July and August 1918. The Allies launch counteroffensives after they stop the German offensive. Battles include the Second Battle of the Marne and the Hundred Days Offensive.
November 9, 1918. Kaiser Wilhelm abdicates.
All of these helped the Allies win. There were many other major battles in this war. Since the Allies did not lose those battles catastrophically, I suppose you could say that they helped the Allies win, but they really did not do the Allies any good in the short term. Major battles include the Battle of the Somme (July to November 1916), the Battle of Verdun (most of the year 1916).