First, I have to know what grade you are in, how old you are and what country you are from? It may be a very valid and good question or not, so if you could tell me.
Dec.7/1941 Japan attacked the United States - Pearl Harbor Naval Base in Hawaii. As a result of that, the US declared war on Japan, and Germany who was their ally, and this finally brought the US into the Second World War.
It was the reason that the US became actively involved in World War II. It was an attack on the United States Naval base at Pearl Harbor by the Japanese Navy. Four U.S. Navy battleships were sunk, 2,402 men were killed and 1,282 more wounded.