In late 1941 war was raging in Europe. Germany and her allies (called the Axis powers) had already defeated most of continental Europe leaving Britain almost alone fighting Germany. Britain was losing.
The United States was not involved in the war directly but they were sending fleets of supplies in merchant ships of England, the US and Canada. These merchant ships were falling under attack by German submarines (called U-boats), so the US Navy was using was using it's destroyers to hunt for and destroy these U-boats.
The American President wanted very badly to become involved with the war. He saw the danger the Axis posed to the world. The American public, however, wanted no part in the war in Europe. Most Americans were related to, or knew people killed in World War I, which had ended only about 20 years before.
In Asia Japan, an ally of Germany was interested in expanding it's own empire. They had already invaded China and Korea. The United States, concerned about the Japanese aggression, was putting political pressure on Japan in an attempt to get them to stop.
Japan wanted to expand their war in Asia and felt that the only thing in their way was the United States Navy. A daring plan was conceived to destroy the US Navy while anchored in their base in Pearl Harbor, Hawaii.
Admiral Isoroku Yamamoto, head of the Imperial Japanese Navy, felt that a war with the United States could not be won. He was overruled by the head of the Army, who had more political influence an Yamamoto. Yamamoto insisted that the US be given a warning of the attack by Japan officially declaring war agents the US before the attack took place. Japan had never done this before but Yamamoto felt that to attack the US without warning would enrage the Americans to the point that they would utterly destroy Japan. Yamamoto knew that the only hope of success was in inflicting so much hurt on the United States that the US would sue for piece rather than fight. If the US was attacked without warning this would never happen.
A declaration of war was sent to the Embassy of Japan in Washington D.C. with instructions to deliver it 30 minutes before the attack took place. The message was sent on the morning of December 7, a Sunday, and the secretarial staff was not present. The Declaration needed to be translated to English and the available staff were not able to do this on time. The Declaration of war was delivered too late.
At about 3 am a midget submarine was spotted near the mouth of Pearl Harbor by a minesweeper, the USS Condor. While looking for this sub, the destroyer USS Ward fired the first American shots of World War II at 6'oclock in the morning, sinking another submarine. Even so, the US Navy leadership did not realize they were under attack.