The Japanese attacked the U.S. naval base at Pearl Harbor in Hawaii at 7:55 am on December 7, 1941. This event brought the U.S. into World War II. America was trying to avoid the war but after the surprise attack on Pearl Harbor by Japan, the U.S. declared war. This event was significant because it led to U.S. involvement in World War II, and created war jobs for many Americans. This helped to end The Great Depression.
Sources: http://www.history.com/photos/pearl-harbor/photo13, http://en.wikipedia.org/wiki/Attack_on_Pearl_Harbor