The US entered World War I for a couple different reasons. One of them was the sinking of the Lusitania. This British merchant ship, carrying hundreds of Americans, was attacked by a German submarine. This attack caused the death of over 100 American citizens.
Along with this, the US also intercepted the Zimmerman Telegram. This telegram was supposed to form a secret military alliance between Germany and Mexico. Germany wanted Mexico to invade the US and promised Mexico they would return California, Arizona, and New Mexico once they won World War I. This angered Americans and caused Congress to declare war on Germany.