Japan and America had a peace treaty, but when the United States began to send assistance to the Chinese during the almost decade long war between Japan and China, Japan continued their aggression with French Indochina, so the United States began to take preventative measures.
In the summer of 1941 the United States had decided to cut of oil exports to Japan, which was over 80% of Japan's oil, and without it's supplies, Japan's navy would not be able to function
Japan, obviously very unhappy with this, decided to break their peace treaty once in place.
Japan then decides to attack the United States on Sunday, December 7th 1941 just before 8 A.M where some 250 aircrafts attacked Pearl Harbor
Eighteen US Naval Vessels, including 8 battleships, were destroyed as well as 300 battleships. This attack also killed 2,403 men.
One December 8th Roosevelt addressed this day was "a date which, later Roosevelt declared war on Japan, and it's allies, Germany and Italy. And 3 days later Germany declared war on the U.S