In 1941 President Roosevelt stationed fifty B17 bombers in the Philippines, standing between the Japanese and Dutch East Indian oil fields they needed so desperately to circumvent the American oil embargo. Roosevelt also ordered the U.S. Fleet to the Pacific. On December 2, 1941, Hawaii received a message from Washington that began, "This is a war warning." From deciphering the Japanese code, the U.S. knew Admiral Yamamoto was planning a Pacific attack, but not when or where. No one believed it would be Pearl Harbor.
A week later that belief would have devastating consequences. In two hours on Sunday morning, December 7, 350 Japanese aircraft sank or badly damaged 21 ships moored at Pearl Harbor, including eight battleships. (All but the Arizona, Oklahoma, and Utah were raised and repaired.) Almost 200 American planes were destroyed, and another 150 damaged. 2,400 Americans died (including 1,177 on the Arizona alone), and Oahu's hospitals were overrun with 1,200 more burned and maimed victims. Besides Pearl Harbor, the Philippines and other American installations were attacked the same day.
The enemy thought destroying the American fleet would lead to Japanese domination of the Pacific for two-years-enough time to build an impenetrable defensive ring using island airfields. But Japanese mistakes would contribute to their eventual defeat. The American aircraft carriers weren't damaged because they were off delivering planes to Midway and Wake Islands, presumed the more likely targets of a Japanese attack. The Japanese also failed to destroy Hawaiian shoreside facilities—including oil storage depots—that would prove vital to the American war effort.
Perhaps the biggest mistake was to attack in the first place. Yamamoto himself said it best: "I fear we will awaken a sleeping giant." On December 8, America was never more awake, and never more vengeful. Roosevelt immediately declared war on Japan (and only Japan), and then got an unexpected dividend. On December 11, Hitler blundered badly by declaring war on the United States, although his agreement with the Japanese didn't require him to do so. That made it easier for Roosevelt to keep his secret promise to Winston Churchill that when the Americans entered the war, they would defeat Germany first.
Historians will never know for sure what prior knowledge Roosevelt had of the Pearl Harbor attack. But they do know that America's emotional response was exactly what the President needed in his battle with the isolationists.