If you are referring to World War I, the sinking of the Lusitania and the Zimmerman Telegram prompted the US to join the allies during World War I. The Lusitania was a British ship that had American passengers on it that was sunk by the Germans. This event, which took place in 1915, resulted in the death of over 100 American citizens. Along with this, the Zimmerman Telegram also caused the US to enter the war. This was supposed to be a secret message from the German government sent to the Mexican government. It was intercepted by the allied forces and decoded. Ultimately, Germany wanted Mexico to invade the US and join their side in World War I. After reading this, the American government was furious and shortly thereafter declared war.
If you are referring to World War II, the US entered after the attack on Pearl Harbor. On December 7, 1941, the Japanese military attack a US military base in Pearl Harbor (Hawaii). This attack harmed/killed thousands of citizens and military personnel. The next day, president Franklin D. Roosevelt asked Congress to declare war on Japan. They voted and were officially into World War II as of December 8th 1941.
