Although the U.S. declared war on Japan the day after Pearl Harbor, and Japan then declared war back, a reasonable person might consider the attack on Pearl Harbor to be a declaration of war, so IMO Japan started it. Germany and Italy declared war on the U.S. on December 11, although I'm not sure exactly why.

Interestingly, the news of the Pearl Harbor attack was heavily censored, with neither accurate casualty figures or number of ships sunk released to the press. Film images of the attack were not shown to the public until January of 1943.

(The press immediately snuck in to take shots of the burning ships and dead sailors and release them to the American people, declaring their "right to know the costs of war."

Just kidding.)