Which event led the united states to enter world war i on the side of the allies

Germany had a plan to try and prevent Britain from getting supplies for war by using unrestricted submarine warfare (the German U boats). At first they only sank Allied ships, but one ship they sunk, a passenger ship called the "Lusitania", held a very large amount of passengers and many of them died. The U.S. threatened to go to war, but Germany backed down and stopped the sinking. After a little while, they thought they could bargain and started up again this time sinking allied ships as well as neutral ships. They thought they could be able to prevent Britain from receiving their supplies before the U.S. joined the war; they were wrong. 2.) Another event leading up to World War 1 was the Zimmerman telegram, a telegram sent from German to Mexico to joint the war on their side and defeat the Americans. Of course, it was caught and decoded (it was in a secret code) and the Americans stopped them. Mexico decided to stay neutral.