Pearl harbor.

The attack time and its consequences

The key elements in Yamamoto's plans were careful preparation, the achievement of surprise, and the use of aircraft carriers and naval aviation on an unprecedented scale. In the spring of 1941, Japanese carrier pilots began training in the special tactics called for by the Pearl Harbor attack plan. An attack force commanded by Vice Admiral Chuichi Nagumo was formed. It centered around six heavy aircraft carriers accompanied by 24 supporting vessels. A separate group of submarines was to sink any American warships which escaped the Japanese carrier force.
Nagumo's fleet assembled in the remote anchorage of Tankan Bay in the Kurile Islands and departed in strictest secrecy for Hawaii on the 26th of November 1941. The ships' route crossed the North Pacific and avoided normal shipping lanes. At dawn the 7th of December 1941, the Japanese task force had approached undetected to a point slightly more than 200 miles north of Oahu. At this time the U.S. carriers were not at Pearl Harbor. On the 28th of November, Admiral Kimmel sent USS Enterprise under Rear Admiral Willliam Halsey to deliver Marine Corps fighter planes to Wake Island. On the 4th of December Enterprise delivered the aircraft and on the 7 th of December the task force was on its way back to Pearl Harbor. On the 5th of December, Admiral Kimmel sent the USS Lexington with a task force under Rear Admiral Newton to deliver 25 scout bombers to Midway Island. The last Pacific carrier, USS Saratoga, had left Pearl Harbor for upkeep and repairs on the West Coast.

Pearl Harbor
Before the attack: reasons and historical background

Pearl Harbor is a harbor on the island of Oʻahu, Hawaii, west of Honolulu. Much of the harbor and surrounding lands is a United States Navy deep-water naval base. It is also the headquarters of the U.S. Pacific Fleet. The attack on Pearl Harbor by the Empire of Japan on the 7th of December, 1941 brought the United States into World War II.
The attack was a surprise intended to remove the US Pacific Fleet as a factor in the war Japan was about to wage against Britain, the Netherlands, and the United States. Two aerial attack waves, totaling 353 aircraft, launched from six Japanese aircraft carriers.
So the aim of the strike was to protect Imperial Japan's advance into Malaya and the Dutch East Indies for their natural resources such as oil and rubber by neutralizing the U.S. Pacific Fleet. Lets take a closer look at this misunderstanding. Both the U.S. and Japan had long-standing contingency plans for war in the Pacific, continuously updated as tension between the two countries steadily increased during the 1930s. Japan's expansion into Manchuria and French Indochina were greeted with steadily increasing levels of embargoes and sanctions by the United States and others. In 1940, under the Export Control Act, the U.S. halted shipments of airplanes, parts, machine tools, and aviation gasoline, which Japan saw as an unfriendly act. Nevertheless, the U.S. continued to export oil to Japan, in part because they understood in Washington that cutting off oil exports would be an extreme step, given Japanese dependence on U.S. oil exports, likely to be taken as a provocation by Japan. In the summer of 1941, after Japanese expansion into French Indochina, the U.S. stopped oil exports to Japan, in part because of new American restrictions on domestic oil consumption. President Franklin D. Roosevelt had earlier moved the Pacific Fleet to Hawaii and ordered a buildup in the Philippines, hoping to discourage Japanese aggression in the Far East.

The problem with the plan was the danger posed by the U.S. Pacific Fleet based at Pearl Harbor. Admiral Isoroku Yamamoto, commander of the Japanese fleet, devised a plan to immobilize the U.S. fleet at the outset of the war with a surprise attack. The attack was one of the most important engagements of World War II. Occurring before a formal declaration of war, it pushed U.S. public opinion from isolationism to an acceptance war was unavoidable, as Roosevelt called the 7th of December, 1941 "... a date which will live in infamy."
So speaking about the background and the events leading to the attack on Pearl Harbor, one has to keep in mind that war between Japan and the United States was, in many ways, inevitable. There had already been a possibility of it since the 1920s, though tension did not begin to grow serious until in 1931 Japan conquered Manchuria. Over the next decade, Japan continued to expand into China leading to all out war between the two in 1937. In 1940, Japan invaded French Indochina, partly in an effort to control supplies reaching China, and partly as a first step to improve her access to resources in Southeast Asia. This move prompted an American embargo on oil exports to Japan, which in turn caused the Japanese to decide to commence the planned takeover of oil supplies in the Dutch East Indies. And once the ceiling of tension was reached it was only a matter of time before war would break out, but the summer and fall of 1941 were needed by both sides to prepare for the inevitable