Japanese planes struck just before 8AM on a Sunday morning. Within a short time five of eight battleships at Pearl Harbor were sunk or sinking, with the rest damaged. Several other ships and most Hawaii-based combat planes were also knocked out and over 2400 Americans were dead. Soon after, Japanese planes eliminated much of the American air force in the Philippines, and a Japanese Army was ashore in Malaya. In so doing the Japanese attempted to knock America out of a Pacific war even before it started. In hindsight, however, the attack was not completely without warning, or even without precedent.
History buffs had long noted the similarity between Pearl Harbor, and Japan's surprise attack on Russian naval squadron at Port Arthur in 1904. In that attack Japan managed to cripple the Russian pacific fleet before Russia could mobilize for war. By 1905 Russia was forced to sign a humiliating peace treaty conceding control of the Korean Peninsula and a strip of northern Manchuria to Japan.
Throughout the first part of the 20th century, the Japanese had been expanding its sphere of influence into Asia and the Pacific. By the early1940s The Japanese military was deeply mired in a war against China and badly needed oil and other raw materials to fuel it's hungry military machine. In response, the Western powers effectively halted trade with Japan in July 1941in an attempt to reign in Japanese aggression in Asia and the Pacific. From then on, as an increasingly desperate Japan schemed to seize the oil and mineral-rich East Indies and Southeast Asia, a Pacific war was virtually inevitable.
In early1940, President Franklin D. Roosevelt had transferred the United States Fleet Headquarters to Pearl Harbor as a deterrent to Japanese aggression. By late November 1941, with peace negotiations clearly approaching an end, informed U.S. officials (and they were well-informed, they believed, through an ability to read Japan's diplomatic codes) fully expected a Japanese attack into the Indies, Malaya and probably the Philippines. Completely unanticipated was the prospect that Japan might attack Hawaii.
Already on the verge of war, the surprise attack effectively ended any last shred of US isolationism and pushed public opinion solidly behind the war effort. The following day President Franklin Roosevelt, addressing a joint session of Congress, called December 7 "a date which will live in infamy." Congress declared war against Japan and it's allies and the country began a rapid transition to a wartime economy, building up armaments in support of military campaigns in the Pacific, North Africa, and Europe.