What are the 3 most important events of World War 1?

Different people can have different opinion on this subject.  My picks for the three most important events of WWI are:

Battle of the Marne.  This battle was fought very early in the war, in September of 1914. Up to this point in the war, the German offensive had been wildly successful.  The Germans, using a version of the Schlieffen Plan, had swept through France and looked like they might capture Paris. This would probably have ended the war. At the Battle of the Marne, the French (with some British assistance) counterattacked.  Their counterattack stopped the German advance. This prevented the Germans from winning the war very easily and caused the war to turn in to the famous stalemate of trench warfare.

Battles of Verdun and the Somme. These battles were fought in 1916 and 1917 over long period of time.  Unlike the Battle of the Marne, these battles did not turn the tide of the war. Instead, these battles killed hundreds of thousands of men without making any difference in the war. Both sides essentially stayed exactly where they were before the battles.  These battles are important events because they typify for us the carnage of the war, where so many died for so little purpose.

Entry of the United States into the war. After years of stalemate, the US entered the war in April of 1917.  The US did not really get enough troops to France to make a difference until 1918. By the spring of that year, there were around 1 million American soldiers in France who could be added to the Allied forces.   Their presence turned the war.  The war had been in a stalemate with neither side strong enough to defeat the other. Throwing a million men into the fight on one side tilted that balance strongly, making Allied victory much more likely. Thus, this was the event that led fairly directly to Allied victory.