A day of infamy: Why Imperial Japan attacked Pearl Harbor on 7 December 1941

The attack on Pearl Harbor is widely considered to be the event that caused the United States to enter the Second World War, leading to the eventual Allied victory over Nazi Germany and Imperial Japan.

Yet in the socially democratic post-war world, it can be easy to forget how incredibly violent the attack and the following Pacific War were.

Its importance as a stepping-stone to the eventual Allied victory and the nuclear age should also not be forgotten.

• Before 9-11, There Was Pearl Harbor
• Pearl Harbor Through Japanese Eyes
• Tora Tora Tora, the living history lesson honouring all who died during attack on Pearl Harbor

What exactly was the Pearl Harbor attack?

It was primarily an air attack on 7 December 1941 unleashed by the Japanese against the US Pacific Fleet, which was stationed in Pearl Harbor in Oahu, Hawaii.

The US and Japan had become competing powers in the years before the Second World War and the attack was intended as a means by which Japan could gain a military and economic advantage.

The Japanese saw it as a necessary next step to prevent them from running out of vital resources, particularly oil.

For their part, the Americans viewed it as an unprovoked and outrageous act of aggression, and they immediately declared war on Japan.

More than 2,000 Americans were killed in the attack, compared with fewer than 100 Japanese, but these deaths were part of a much larger conflict already raging around the globe.

The Second World War started in Europe when Germany invaded Poland in 1939.

But the Japanese had already begun an aggressive military expansion in the Pacific, which included the attack on the Chinese capital of Nanjing in December 1937.

How did the attack take place?

For both sides, the air assault on Pearl Harbor was chaotic.

It almost began as an American-initiated naval battle rather than a Japanese air assault on the US Pacific Fleet.

Early on the morning of 7 December 1941, the USS Ward was patrolling off the entrance to Pearl Harbor when it fired at and dropped depth charges on a Japanese submarine.

It had spotted the sub's conning tower and radioed for it to make itself known. When it did not, the Ward attacked and sank it.

The sub was at the forefront of the attack which would shortly take place, and Japanese planes were already scouting ahead in the skies above.

The Japanese reconnaissance planes had spotted their enemies and according to Carl Smith in 'Pearl Harbor 1941 The Day of Infamy', they radioed back exactly what they saw, saying: "Enemy… at anchor, nine battleships, one heavy cruiser, six light cruisers."

The American warships were sitting ducks for the recently developed Japanese torpedoes that were able to strike ships in shallow water - just 33 feet at Pearl Harbor.

Yet confusion almost took hold before the attack had begun, with the commander of the following attack planes releasing flares to indicate that the attack should commence, as not all the Japanese planes had radios.

One flare was meant to be the signal for the torpedo bombers to attack first and catch the Americans by surprise, while two flares were a signal for the regular bombers to go in and cause additional damage.

The formation commander fired two flares, but only because he thought an initial delay meant his first flare had not been seen.

Instead, the bombers took this as their cue and swooped into action, though by now, realising what had happened, the torpedo plane pilots did likewise.

Mass carnage followed on the ground and in the harbour as Japanese planes strafed roads and bombed buildings, aircraft hangers and airfields.

Smith describes how lines of American aircraft caught fire and how this then spread along the path of leaking aviation fuel, causing a chain reaction of explosions that eventually destroyed many of the planes.

In the harbour, torpedoes and bombs hit home, sending burning petrol and oil across the water and thick smoke wafting up into the sky, followed by several explosions.

The USS Arizona was particularly badly hit, and Smith describes the scene this way, saying: "Arizona's explosion knocked men off nearby vessels due to the might of the concussion.

"The bomb pierced her forward magazine and the explosion was so powerful that damage control parties aboard nearby [USS] Vestal were blown overboard when a fireball erupted skyward."

Why did Japan attack Pearl Harbor?

The circumstances surrounding the attack are complex, though essentially there are two main reasons.

The first was that Japan had been in isolation for hundreds of years until the middle of the 19th Century when it was visited by American ships interested in opening up trade networks.

This proved to be a catalyst for rapid Japanese modernisation.

The second and larger factor was the start of the Second World War, driven in part by the global economic shock of the Great Depression, something that was felt particularly acutely within Japan.

As the documentary series The World at War pointed out, part of Japan's difficulty was that the material shortages had coincided with rapid population growth, leading to an additional 1,000,000 extra mouths to feed a year.

Additionally, Japan had no mineral resources of her own, unemployment was high and crop failures brought disastrous famine in rural areas.

In the face of these desperate circumstances, militarisation followed.

The Japanese expanded across both China and Indochina, but this brought them too close to the US-controlled Philippines.

American embargoes followed on iron ore, aviation fuel and then oil.

The Japanese decided that if the situation could not be resolved diplomatically, a military strike on Pearl Harbor would be necessary. This was to be part of a larger strike on the Dutch East Indies to secure supplies of oil.

Attacks on British-held Malaya and Singapore, as well as Pearl Harbor, were deemed necessary to prevent the US Navy and Royal Navy from blocking the movement of the oil back to Japan.

Why was Pearl Harbor so important?

The obvious answer is that it brought the US into the Second World War, helping the Allies achieve ultimate victory.

In a sense, it therefore helped to usher in the democratic and relatively peaceful post-war world.

Without US involvement in the European Theatre, had the USSR proved victorious against Germany without as many, or any, concurrent attacks on Germany by the Western Allies, post-war Europe would almost certainly have been more communist-influenced than it ended up being.

However, it was the declaration of war on the United States by Adolf Hitler on 11 December 1941 following the Pearl Harbor attack that brought the US into the European Theatre.

The Pearl Harbor attack also unleashed the Pacific War between the US and Japan, which turned into a particularly bloody conflict.

It ended in part following the dropping of the atomic bombs on the Japanese cities of Hiroshima and Nagasaki.

Thus, Pearl Harbor was the event that led to some of the worst events of the Second World War, while also being important to the war's outcome and the eventual Allied victory.

This had already occurred in May 1945 in the European Theatre.

The Japanese surrender happened after the dropping of the atomic bombs, with the Japanese accepting the surrender terms on 14 August 1945 and messaging their embassies around the world to that effect.

It was announced the following day on 15 August in Japan itself and on the evening of 14 August in the US, due to the time difference.

VJ Day is celebrated on 15 August in Britain and on 2 September in the US, which was when the official surrender ceremony took place on board the USS Missouri.