There were several different reasons why the US entered WWI. One of the biggest ones that was public was the sinking of the Lusitania. This British ship, that carried American passengers, was entering what was considered a war zone at the time. Even though the Lusitania was entering a war zone, it was a passenger ship (meaning it had nothing to do with the military). Despite this fact, the Germans still used submarine warfare in order to destroy it. The Lusitania sank, killing over 100 Americans. This public event caused American citizens to be angry with Germany and their use of submarine warfare.
Along with this, the public reveal of the Allied powers interception of the Zimmerman Telegram also lead US into World War I. This telegram was supposed to be a secret message in which an alliance was formed by Germany and Mexico. Germany wanted Mexico to invade the US and help them win World War I. Once intercepted, this became public knowledge as to what Germany's plans were and resulted in the US officially declaring war shortly thereafter.
