Pearl Harbour

At Pearl Harbour on 7th December 1941the USS Arizona exploded and sank after being attacked by Japabnese aircraft
On the morning of 7th December 1941 the Imperial Japanese Navy launched an attack, codenamed operation Z, on the American Naval base at Pearl Harbour in the Hawaiian islands of the Pacific Ocean. The attack was a pre-emptive strike at the United States Pacific Fleet intended to remove the possibility of the US Navy from interfering with Japanese plans to conquer parts of South East Asia and Indonesia.
The attack was conducted by over 350 Japanese fighters, bombers, dive bombers and torpedo bombers launched from six aircraft carriers. It resulted in four American battleships being sunk with another four being damaged. Two of the sunk battleships were later raised and returned to service. Three cruisers and several smaller vessels were also sunk and 188 aircraft destroyed. Over 2,000 US personnel were killed, over 1,000 being from the USS Arizona alone, and over a 1,000 were injured. Japanese losses were limited to 29 aircraft, under 100 personnel killed and the loss of a few midget submarines.
The Japanese considered the attack to be very successful in that it prevented short term operations by the US Navy against troops involved in campaigns to secure what they considered their primary targets: the oilfields and rubber in South east Asia.
The attack, however, failed to inflict any serious damage on critical installations and the harbour facilities were soon back in action. Most importantly the Aircraft Carriers were not in port that morning and as a consequence were available for immediate deployment against the Japanese fleet.
The attack led to the USA declaring war on Japan and, with Germany supporting Japan by declaring war on the USA, finally allowed the USA to formally and openly support the Allies in their war against the Axis powers.

Background

In 1931 Japan invaded and conquered the Chinese province of Manchuria. The Japanese then attempted to conquer the rest of China. This conflicted with American interest in the area and so the US decided to strengthen their military forces in the Pacific, especially in the Philippines, and to move the Pacific fleet from San Diego on the West Coast of America to the Hawaiian Naval base at Pearl Harbour in the middle of the Pacific. The US also imposed a trade embargo on Japan, specifically concerning oil.
These measures were intended to curb Japanese ambitions in the far east but in fact served to infuriate the Japanese government led by a dominant military faction. The oil embargo threatened to jeopardise Japanese plans for Chinese domination and they looked for an alternative source of oil. The oil fields in Java were a tempting target and with Britain, France and other European nations occupied with the war against Germany Japan felt that it could easily take them. The major obstacle to this was the USA, in particular the battleships of the Pacific Fleet located at Pearl Harbour.
The Japanese reasoned that if they could quickly seize the resources they wanted, knock out the battleships of the Pacific fleet and establish a strong defensive perimeter through the islands around Japan then America could be persuaded to negotiate a peace on Japan's terms. To strengthen their position in 1940 they aligned themselves with the Axis powers in Europe.

Planning

The Japanese Mitsubishi A6M Zero fighter was used to secure air superiority of Pearl Harbour
Probably the best fighter at that time: the Japanese Mitsubishi A6M
It was felt that the best way of removing the Pacific Fleet would be to carry out a surprise attack on the fleet at anchor in Pearl Harbour.
The task of planning the attack fell to Admiral Yamamoto Commander in chief of the Imperial Joint Fleet. His plan called for a large carrier force delivering over 300 aircraft to a point off Hawaii from where they could attack the Naval base and surrounding air bases on the island of Oahu with bombs and torpedoes. It was a bold plan that required a large fleet to travel thousand's of miles across the Pacific in total secrecy to achieve complete surprise over the US base.
Despite serious objections by the Naval HQ Yamamoto's plan was approved by the Imperial Naval General Staff in October 1941, by Emperor Hirohito in November and given final authorisation in December.

Training and technology

Curtiss P40 was a World War Two American Fighter
3 American Curtiss P40's managed to engage the Japanese aircraft claiming 7 dive bombers.
The depth of water at Pearl Harbour was about 14m (45 ft) . Torpedoes of the time needed about 23m (75ft) of water to operate.
The Japanese analysed a successful attack by Royal Navy Swordfish torpedo bombers on the Italian Fleet at anchor in Taranto during November 1940.
From this study they concluded that an attack at Pearl Harbour to be feasible and set about designing modifications to allow their torpedoes to operate in shallow water.
When dropped a torpedo assumed a nose down attitude. When it entered the water there was a tendency for it to plunge and, if there was insufficient depth for the onboard gyro to adjust for a level trim, it would hit the ground and stick or explode. Wooden fins were added to the torpedo to keep the nose up in descent and ensure a shallow entry to the water. The fins fell off due to the force of impacting on the water.
Another problem was that when released the torpedo could start to roll and upon entry to the water the stabilising rudders would be in the wrong position sending the torpedo out of control; veering away from the target, diving, skipping out of the water etc. The solution that the Japanese engineers came up with was to fit anti-roll rudders that detected when the torpedo was rolling and applied movement in the opposite direction.
These modifications allowed the pilots to drop their torpedoes on target in as little as 10m of water and in heavy seas. This modified Type 91 torpedo design was available by August 1941.
Regardless of the problems with the torpedoes Japanese carrier pilots were being trained as early as Spring 1941 in special tactics specifically devised for launching torpedoes against anchored ships in the shallow water of Pearl Harbour.

The Attack

The Japanese Kido Butai (strike force) consisting of six carriers (Akagi, Kaga, Sōryū, Hiryū, Shōkaku, and Zuikaku) and an escort of twenty four supporting vessels left a remote northern base in the Kurile islands on 26th November 1941. The force avoided normal shipping lanes and by 7th December had arrived undetected 200 miles off the northern tip of the Hawaiian islands.
By this time the three American aircraft carriers had departed Pearl Harbour: USS Enterprise was delivering aircraft to Wake Island, USS Lexington was delivering aircraft to Midway and USS Saratoga was en-route to the West Coast for repairs and overhaul.
The Akagi was one of six fleet carriers used by the Japanese at Pearl Harbour.
At 6-00 am the first wave of 181 aircraft, composed of bombers, dive bombers, torpedo bombers and fighters launched from the carriers. Organised in three groups they arrived over the island of Oahu just before 8-00am achieving complete surprise.
As attacks began on the shipping at Pearl Harbour Group 2, consisting of 54 Aichi D3A 'Val' dive bombers, attacked the airfields at Ford Island Naval Air Station, Hickam Field, Bellows Field, Wheeler Field, Kaneohe Naval Air Station and Ewa Marine Corps Air Station. Parked aircraft were strafed and bombed to prevent them interfering with the attacks on Pearl Harbour. Group 3, consisting of 45 Mitsubishi A6M Zero fighters, was tasked with providing air cover for the other two groups. The primary targets for Group 1, consisting of 50 Nakajima B5N 'Kate' bombers and 40 'Kate' torpedo bombers, were the eight battleships: seven of which were anchored in Battleship row and one, USS Pennsylvania, was in dry dock. The attack was executed quickly and within about 30 minutes all seven battleships in battleship row had been hit by bombs or torpedoes: USS West Virginia (BB-48) had been sunk; USS Oklahoma (BB-37) had capsized and sank; USS Arizona (BB-39) was sunk by an armour piercing bomb which exploded ammunition in the forward magazine killing 1,177 and injuring around 1,000 crewmen; USS Tennessee (BB-43), USS Maryland (BB-46), USS Nevada (BB-36) and USS California (BB-44) were damaged.
The Japanese 1st wave approched Pearl Harbour from various directions
There was a short lull in which USS Nevada, despite being battered, managed to get underway and attempted to reach the open sea when the second wave of 170 aircraft, launched at 6-30 am, arrived. USS Nevada received several hits and was beached rather than risk being sunk in the narrow entrance channel blocking it.
Japanese second wave attacked installations at Pearl Harbour from the East and South
Japanese junior officers urged for a third wave to destroy installations, fuel and munitions but Admiral Nagumo, the strike force Commanding officer, decided against it on several grounds: that American defences were now alerted and had caused most of the Japanese casualties during the second wave; fuel was running low; the American carriers were still at large; the third wave would have to land at night. Nagumo considered his task complete and decided to conserve his force for future operations.
The attack had lasted two hours and resulted in the loss or damage of twenty one ships of the American Pacific Fleet, 188 aircraft destroyed and 159 damaged (mostly on the ground), 2,403 dead and 1,178 wounded. Of the Battleships hit only three were not recovered: USS Arizona was damaged beyond repair and was left as a memorial to those that had died, USS Oklahoma was raised but not thought worth repairing and USS Utah was considered to be too old and obsolete to be worth raising.

Midget Submarines

Five Japanese submarines, each carrying a two man 'Ko-hyoteki' midget submarine departed Kure Naval base on 25th November 1941. In the early hours of December 7th they launched the midget-submarines, which proceeded to gain access to Pearl Harbour. One midget was sunk by USS Ward in the harbour entrance. One midget missed the Seaplane tender Curtiss with her first torpedo and missed USS Monaghan with the other before being sunk by USS Monaghan. One midget HA-19 ran aground and was subsequently captured. A fourth was damaged and was abandoned by its crew. One midget is thought to have fired torpedoes at Battleship row and was then scuttled, the crew being recovered in a pre-arranged way somehow.

The Aftermath

Due to delays at the Japanese Embassy a document containing hints at war on the United States and the British Empire wasn't delivered until after the Pearl Harbour attack. This was interpreted as a deliberate act to achieve surprise. The American response was their own declaration of war on Japan. On December 8th at 12:30 pm, Roosevelt addressed Congress.

"Mr. Vice President, and Mr. Speaker, and Members of the Senate, of the House of Representatives: Yesterday, December 7th, 1941, a date which will live in infamy, the United States of America was suddenly and deliberately attacked by naval and air forces of the Empire of Japan. The United States was at peace with that Nation and, at the solicitation of Japan, was still in conversation with its Government and its Emperor looking toward the maintenance of peace in the Pacific. Indeed, one hour after Japanese air squadrons had commenced bombing in Oahu, the Japanese Ambassador to the United States and his colleague delivered to the Secretary of State a formal reply to a recent American message. While this reply stated that it seemed useless to continue the existing diplomatic negotiations, it contained no threat or hint of war or armed attack. It will be recorded, that the distance of Hawaii from Japan makes it obvious that the attack was deliberately planned many days or even weeks ago. During the intervening time the Japanese Government had deliberately sought to deceive the United States by false statements and expressions of hope for continued peace. The attack yesterday on the Hawaiian Islands has caused severe damage to American naval and military forces. Very many American lives have been lost. In addition American ships have been reported torpedoed on the high seas between San Francisco and Honolulu. Yesterday the Japanese Government also launched an attack against Malaya. Last night Japanese forces attacked Hong Kong. Last night Japanese forces attacked Guam. Last night Japanese forces attacked the Philippine Islands. Last night the Japanese attacked Midway Island. Japan has, therefore, undertaken a surprise offensive extending throughout the Pacific area. The facts of yesterday speak for themselves. The people of the United States have already formed their opinions and well understand the implications to the very life and safety of our Nation. As Commander-in-Chief of the Army and Navy I have directed that all measures be taken for our defence. Always will we remember the character of the onslaught against us. No matter how long it may take us to overcome this premeditated invasion, the American people in their righteous might will win through to absolute victory. I believe I interpret the will of the Congress and of the people when I assert that we will not only defend ourselves to the uttermost but will make very certain that this form of treachery shall never endanger us again. Hostilities exist. There is no blinking at the fact that our people, our territory, and our interests are in grave danger. With confidence in our armed forces - with the unbounded determination of our people - we will gain the inevitable triumph - so help us God. I ask that the Congress declare that since the unprovoked and dastardly attack by Japan on Sunday, December seventh, a state of war has existed between the United States and the Japanese Empire. Franklin D. Roosevelt, The White House December 8th, 1941"

Later at 4-00pm Roosevelt signed the declaration of war.

"Joint Resolution declaring that a state of war exists between the Imperial Government of Japan and the Government and the people of the United States and making provisions to prosecute the same. Whereas the Imperial Government of Japan has committed unprovoked acts of war against the Government and the people of the United States of America: Therefore be it Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, That the state of war between the United States and the Imperial Government of Japan which has thus been thrust upon the United States is hereby formally declared; and the President is hereby authorized and directed to employ the entire naval and military forces of the United States and the resources of the Government to carry on war against the Imperial Government of Japan; and, to bring the conflict to a successful termination, all of the resources of the country are hereby pledged by the Congress of the United States. Approved, December 8, 1941, 4:10 p.m. E.S.T."
The US lacked the resources to oppose the immediate Japanese expansion, but due to the enormous industrial capacity of the United States the damage at Pearl Harbour was quickly restored.
Before the attack the fleet carriers, and submarines, had been of minor strategic importance. After the attack they were the only major elements of the Navy available and tactical and strategic doctrine evolved around them. Ironically it was these assets, and teh opportunities they presented, that provided the most significant contribution to Japan's defeat.
The spirit in which the attack on Pearl Harbour had been conducted enraged the American public and ensured that the Japanese aims of a quick negotiated peace were not realised. The only terms that the United States and Britain would accept was total 'Unconditional Surrender' and that was the policy that was pursued until Japan finally surrendered in 1945. On December 11th, Japan's allies, Germany and Italy, also declared war on the United States. Congress responded with a declaration of war on both those nations.