Japan attacked the U.S Pacific Fleet at its base at Pearl Harbor on the 7th of December 1941, but what led to that decision? Why did the Japanese attack the USA? - The answer is oil.

Japan had been modernising its economy throughout the 20th century and wanted to build an empire of its own. However, Japan lacked the natural resources to make it a reality, with all but 6% of its oil supply being imported. After capturing Manchuria, Japan became bogged down in a full-scale war with China in 1937 and had to look elsewhere for the resources it needed to fight. Meanwhile, the USA was slowly awakening from its isolationism.

When Japan occupied French Indochina in 1941, America retaliated by freezing all Japanese assets in the states, preventing Japan from purchasing oil. Having lost 94% of its oil supply and unwilling to submit to U.S demands, Japan planned to take the oil needed by force. However, striking south into British Malaya and the Dutch East Indies would almost certainly provoke an armed U.S response. To blunt that response, Japan decided to attack the U.S Pacific Fleet at Pearl Harbor, hoping that the U.S would negotiate peace.

The attack at Pearl Harbor was a huge gamble, but one which did not pay off. Though Japan took its objectives in the Pacific and Southeast Asia, the U.S did not respond as expected. Instead of reverting to isolationism, the U.S geared up for total war and Japan's fate was sealed.

Japanese strategy explained

In this episode of IWM Stories, Adrian Kerrison looks at why the Japanese decided to attack the U.S Pacific Fleet at Pearl Harbor.

On the 26th of November 1941, a Japanese attack fleet consisting of six aircraft carriers, two battleships, and hundreds of aircraft departed from Japan and began the long journey to an assembly point 230 miles north of the Hawaiian island of Oahu. Their target, the U.S Pacific Fleet anchored at Pearl Harbor. Scheduled for the 7th of December, the attack would take the Americans by complete surprise paralyzing their fleet for months and costing thousands of lives. However, the attack would also change the course of the Second World War and spell ultimate doom for Imperial Japan. So why did the Japanese attack Pearl Harbor in the first place and how did Japanese miscalculation in planning the attack doom them to defeat in the Second World War? Well to answer that question we first need to go back to the 1930s.

So Japan had spent much of the early 20th century modernizing its economy and its military. So basically they wanted to build an empire sort of like that of Great Britain the United States and from that they could extract natural resources, exploit labor and build new trade routes and become one of the world's great powers.

But while japan had big ambitions there was one huge problem, the Japanese mainland did not have the natural resources required to build that empire. Japan needed to get its hands on more coal iron and, in particular, oil to make its ambitions a reality.

It was 1931 when Japan took its first major step towards empire-building, invading the Chinese province of Manchuria. Now Manchuria had many of the resources that Japan needed and gave them a firmer foothold on the Asian continent for future advances. Over the next few years, Japan poked and prodded its way further into northern china before all-out war broke out between the two in July 1937.

At first, things went very well for the Japanese, they won victory after victory. All the while carrying out major atrocities like the rape of Nanking and the terror bombing of Chinese civilians which drew widespread international condemnation. By 1939 though the war had descended into a stalemate and as the Chinese grew in strength, the war became a serious drain on Japanese manpower and supplies. To win they would have to look elsewhere for the resources they needed.

Meanwhile across the Pacific, the U.S was looking on with mounting concern.

After the U.S.A's participation in the First World War they start to adopt an unofficial policy of non-interventionism and isolationism. So this basically means that they won't go to war for their allies, or even get into alliances in the first place, and they won't even provide aid either. And this actually starts to become official policy in the mid-1930s when the U.S congress starts to pass a series of neutrality acts. But as congress was passing these acts the world around the U.S was getting a lot more violent and unstable.

So though America had began the 1930s as a bastion of isolationism, the outbreak of war in Europe, as well as Japanese atrocities in China, brought a gradual shift in public opinion back towards interventionism. That allowed U.S President Franklin D. Roosevelt to sign a new neutrality act into law in 1939 which permitted the U.S to supply arms to Britain and France if they paid for and picked it up in their own ships. This would later be followed up by the far more sweeping Lend-Lease in 1941 which included China and the Soviet Union and asked for no payment in return. So although the U.S was still technically neutral, it was very clear whose side they were on and for Japan that was a huge problem.

So the biggest resource that Japan needs at this point is oil. In 1939 all but 6% of its oil supply was imported with roughly 80% coming from the United States alone. Running out of oil would basically spell doom for their military campaign in China as well as their other territorial ambitions. There were also a host of other natural resources that Japan needed, but could only get through imports and that included scrap metal, coal, iron all things that are vital to their war effort and actually a lot of this stuff also comes from the United States.

To get those resources and grow its empire Japan had a choice to make between what became known as the northern and southern strategies. The northern strategy was backed by the Imperial Japanese Army and involved taking the oil, coal and iron-rich areas in China, Mongolia and Siberia. The southern strategy on the other hand was backed by the Imperial Japanese Navy and instead involved striking south into British Malaya and the Dutch East Indies, similarly rich in oil and rubber. By the mid-1930s the northern plan was already in full swing with attacks in Manchuria and China and this had led to border disputes with the Soviets. These culminated in the huge Battle of Khalkhin Gol which the Soviet-Mongolian forces won a major victory. Suddenly Japan had to reconsider its plans.

So japan's defeat at Khalkhin Gol basically pours cold water on their plans for northward expansion in Siberia, as does the signing of a non-aggression pact between the Soviet Union and Germany in August 1939. When Germany invades the Soviet Union during Operation Barbarossa in June 1941 these plans for an invasion of Siberia are briefly reconsidered. But Japan is bogged down in China, they're running out of natural resources and it just doesn't happen.

With the army bog down in China it was the navy who took up the mantle as Japan focused on its southern strategy instead. This began in earnest in 1940 when, in order to cut a key Chinese supply route, Japan entered the northern parts of French Indochina in an agreement with the Vichy French government. This worked in isolating the Chinese, but the U.S saw it as yet another act of Japanese aggression that threatened U.S interests in the Pacific. Coupled with Japan's recent alliance with Nazi Germany and Italy, the U.S responded by imposing an embargo on iron, steel, and copper all of which were essential to Japan's war industries and which were largely imported from the U.S. But the Japanese did not learn their lesson and occupied even more of French Indochina in July 1941 as a launch point for invasions further south. This time the Americans responded even more forcefully.

So this time the U.S responds by freezing all of Japan's assets in the United States and this prevents Japan from purchasing oil. And right after this is followed up by Britain and the Netherlands who control the Dutch East Indies imposing oil embargoes of their own. So in one fell swoop, Japan loses 94% of its oil supply.

Japan was in a crisis. They first attempted to negotiate with the U.S who demanded their immediate withdrawal from China and the Tripartite Pact. But for Japan accepting those demands was akin to complete defeat. Unwilling to give up their imperial ambitions, the Japanese felt their only option was to seize the natural resources they needed by force. That meant striking further south into British Malaya and the Dutch East Indies who were both friendly with the U.S. Japan believed that this time the U.S would almost certainly respond to their invasion with force of their own. The Japanese decided then that they had to blunt that U.S response by attacking the U.S Pacific Fleet at anchor Pearl Harbor in Hawaii.

By attacking Pearl Harbor Japan believes that it can severely cripple the U.S fleet and buy them time in the Pacific and Southeast Asia. So not only would they be able to launch their attacks without interference from the U.S they would also have time to dig in defensively and consolidate their gains. So this is a really big gamble for Japan, they don't actually believe that they can win a long-drawn-out war with the U.S so their strategy really hinges on a short war. They believe that the U.S probably won't have the stomach to fight this costly war against a dug-in enemy thousands of miles away across the Pacific and they would instead negotiate for peace. Allowing japan to retain some or all of its captured territories.

On December 7th 1941 those plans were finally put into action. At 7:55 am the first attack wave of 183 aircraft appeared in the skies over Pearl Harbor, the Americans were taken completely by surprise. The wave was separated into three groups. The first two groups of dive bombers and fighters targeted the hangars and parked aircraft of the island's airbase. The aircraft there were stored wingtip to wingtip to prevent sabotage, but that made them easy pickings for the Japanese. The other group of bombers and torpedo bombers targeted the ships in the harbour, in particular, the battleships on 'Battleship row'. The Americans believed that the water was too shallow for a torpedo attack, but the Japanese had created a brand new kind of torpedo specifically designed for the waters of Pearl Harbor and it had a devastating effect.

Within the first five minutes of the attack, four battleships were hit including the USS Oklahoma and the USS Arizona which exploded 10 minutes later killing 1,175 of its crew.

At 8:54 am the second attack wave of 170 aircraft began their attack. They were also separated into three groups attacking mostly the same targets, but with the base now on high alert, their attacks were less successful.

In the space of just over an hour, the Japanese had sunk or damaged 18 American warships, including hits to all eight of the fleet's battleships. They destroyed 188 aircraft and severely damaged the base's infrastructure. Crucially though, the three all-important U.S aircraft carriers were out on manoeuvres at the time of the attack and escaped unscathed.

So because Japan are sort of anticipating this short war that's going to lead to negotiations their target selection focuses on the battleships which are going to prevent the U.S Pacific Fleet from coming out into Pacific and Southeast Asia and stopping the Japanese and they're not thinking about things like the fuel depots and the repair shops that are actually going to allow America to pursue a longer war in the Pacific. The shallow depths meant that any ships that sunk, they didn't sink far down so they were much easier to recover. Almost half of the deaths that day on the U.S side were from the USS Arizona when it was hit and exploded and the Imperial War Museum in London actually has a piece of the USS Arizona on display in its new Second World War galleries and this is actually the first time that part of the USS Arizona has been displayed outside of the United States.

Initially, the attack worked perfectly. On the same day the Japanese launched more or less simultaneous attacks in the Pacific and Southeast Asia. Thailand surrendered within hours and quickly signed an alliance with Japan, while the U.S territories of Guam, Wake Island and the Philippines as well as the British territories of Malaya and Hong Kong all fell relatively quickly. And on top of that two major British warships the HMS Prince of Wales and HMS Repulse were sunk off the coast of Malaya by Japanese torpedo bombers.

In the first months of 1942, Japan followed this up with attacks on the Dutch East Indies, British Burma and Singapore, New Guinea and the Solomon Islands and just as they'd hoped the U.S Pacific Fleet was unable to offer a response. The Japanese then had completed their goal with speed and efficiency. They'd established their new empire and they finally had the natural resources they'd craved for so long. But there was one huge problem.

So Japan's attack on Pearl Harbor pretty much has the opposite effect of what it was hoping for, you remember they were hoping for this negotiated peace. So the day after the attack, President Roosevelt delivers his famous 'day of infamy' speech to congress in which he asked for a formal declaration of war against japan which congress quickly authorizes. So the U.S is officially now in the war.

The vast resources of the United States power, raw materials, industrial production all had to be mobilized to meet the demands of total war.

So support for isolationism quickly melts away, there's a rapid expansion of the U.S military with hundreds of thousands of men volunteering to join and the economy is fully mobilized onto a war footing. Japan's hopes for a short war completely evaporate and they've now awoken this what many people call 'sleeping giant' and they're now committed to this long war in the Pacific and Southeast Asia which ultimately they'll lose.

The Japanese attack at Pearl Harbor was a huge gamble and one which did not pay off. Japan's desire for an empire and the natural resources to go with it had slowly awoken the U.S from its isolationism. Bogged down in China and unable to attack the Soviets the Japanese decision to strike south resulted in a U.S oil embargo which gave Japan little choice other than to give up its ambitions or go to war. Their decision to fight paid off in the short term, but once the U.S had geared up its war machine for Japan there was little hope of victory.

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