A gathering storm: why did Japan attack Pearl Harbor in 1941?

• Jump to our timeline of Japan's preparation for the attack

The Japanese army’s Manchurian ruse fooled nobody. On 22 September 1931, the US secretary of state, Henry Stimson, cabled the League of Nations: “It is apparent that the Japanese military have initiated a widely extended movement of aggression only after careful preparation…” Unbeknown to the world, what would come to be called the ‘Mukden Incident’ would lead inexorably towards Pearl Harbor. The Stimson Doctrine that followed – the US government’s policy of refusing to recognise states created by force – was essentially a letter to the Japanese government telling it to keep its hands off China – a country that had, until the 1820s, been Asia’s greatest power, but was now riven by civil war and threatened by Japanese and Soviet plans to dismember it.

Japan’s grievances

In January 1919, when the great powers met for the Paris Peace Conference that marked the settlement of the First World War, Japan joined the top table alongside the United States, Britain, France and Italy. Nevertheless, the Asian country still felt like an outsider.

Japan entered the negotiations with two principle aims. Firstly, it wanted a clause on racial equality inserted into the Treaty of Versailles. This demand stemmed from the passage of a series of anti-Japanese race laws in the US, culminating in the California Alien Land Law of 1913, which banned immigrant farmers from owning land. Racist treatment of Japanese businessmen was also seen as endemic in Singapore, Hong Kong, Shanghai and other western imperial outposts. So strong was the feeling of racial conspiracy that Duke Aritomo Yamagata, the statesman who had modernised the Japanese army after the overthrow of the shogunate in 1868, warned that it was “extremely important… to take steps to prevent the establishment of a white alliance against the yellow people”. However, the western powers quashed the demand for a racial clause in the Versailles treaty – a decision that would feed into Japan’s ultra­nationalist narrative in years to come.

Japan’s second demand – for the permanent transfer of Germany’s imperial assets in Asia – was only slightly more successful. Having seized Germany’s Kiautschou Bay concession, a valuable 213-square-mile territory on China’s eastern seaboard, Japan was forced to hand it back to China. As for Germany’s southern Pacific empire, Japan was strongarmed into sharing the spoils with Britain and Australia.

Greater insults to Japan’s national dignity soon followed at the Washington Naval Conference, which began in November 1921. The aim was to create a multilateral arms limitation treaty by restricting the building of battleships – the weapons of mass destruction of their day. The outcome, which limited the US and Britain to 525,000 tons each while Japan was restricted to 315,000 tons (a ratio of 5:5:3), did little to convince Japanese ultranationalists that the Anglo-Saxon countries were playing fair. In response to the Washington Naval Treaty, Kametaro Mitsukawa, an influential nationalist intellectual, claimed the western powers were “plotting to subjugate Asia completely by the end of the 20th century”.

A democracy in retreat

In spite of these setbacks, Japanese mainstream politicians continued to support the global postwar settlement, whose twin pillars were the League of Nations and the Washington Naval Treaty. However, Japan’s democratic institutions, which had worked well since the overthrow of the Tokugawa feudal dictatorship in 1868, and its later replacement by the Meiji Constitution – an ill-defined mix of constitutional and absolute monarchy – were ultimately undone in the early 1930s. The Great Depression undermined Japan’s democratic constitution. Factions in the armed forces and the press took advantage of popular discontent to push their nationalist and anti-capitalist agendas. Moreover, the young Emperor Hirohito refrained from using the supreme powers given to him by the Meiji Constitution to push back against the military.

On 14 November 1930, Prime Minister Osachi Hamaguchi was shot and seriously wounded as he entered Tokyo’s main railway station. His health would never recover, and he died eight months later. His attacker was a member of the ultranationalist Aikokusha (‘Society of Patriots’) party, one of a rash of such groups that sprung up in 1920s Japan. The prospect of an economy sliding into depression, combined with widespread hostility to the London Naval Treaty of 1930 – which renewed Japan’s disparity with the west – meant that the attack elicited little public opprobrium.

Eighteen months later, on 9 February 1932, a former finance minister was gunned down by a student member of the ultranationalist Ketsumeidan (‘League of Blood’). Next Baron Takuma Dan, a western-sympathising graduate of the Massachusetts Institute of Technology, was murdered outside the offices of Mitsui Bank, where he was an executive. It was no coincidence that Baron Dan had just hosted the visit of the Earl of Lytton, chairman of a League of Nations committee investigating the Mukden Incident. Worse was to follow. On 15 May 1932 another anti-capitalist group, this time made up of junior naval officers, organised hit squads on eminent liberal figures. Prime Minister Inukai was murdered at home; his house guest, Charlie Chaplin, the legendary Holly­wood comic star, was also targeted, as a famous westerner. Chaplin was lucky to be out watching Sumo wrestling with Inukai’s son. In a sign of Japan’s increas­ingly nationalist mood, instead of being condemned, the assassins were popularly cel­ebrated and given short prison sentences.

The political consequences of the assassination were far-reaching. Rather than choosing Inukai’s successor from the majority Seiyūkai party, the emperor appointed a navy stalwart, Admiral Viscount Saitō Makoto, as head of a unity government. From this point on, the political parties withered into insig­nificance, before they were abolished in 1940 when Prime Minister Fumimaro Konoe accepted their ‘voluntary’ liquidation and absorption into his Imperial Rule Assistance Association – an attempt to create a one-party state on the model of Germany’s Nazi party.

Tipping point

Increasing opposition to liberals, capitalists and internationalists reached its apogee on 26 February 1936, when 19 young army officers launched a coup d’état aimed at protecting the emperor and preserving Japan’s kokutai (‘national essence’). Their views reflected a military education based on the Imperial Rescript for Seamen and Soldiers, an 1882 code of ethics that all personnel were required to memorise, which promoted their mission as guardians of a ‘sacred nation’ and their absolute personal loyalty to the emperor. This 2,700-kanji [character] document was a potent example of brainwashing, marrying bastardised samurai values based on bushido (‘the way of the warrior’) with modern weaponry and training.

Many of the Japanese army’s senior generals, also infused with the mythical cult of the god-emperor, gave tacit support to the young officers. The emperor himself, however, was furious. Unlike in 1932, the rebel officers had sought to overthrow the government itself. Hirohito ordered them to be tried, found guilty and executed. So much for the Japanese postwar myth that he was a powerless constitutional monarch.

However, instead of moving to contain the power of the army after the coup attempt, Hirohito allowed it to entrench its political position. In May 1936, the law was changed to allow only active generals and admirals to fill the post of minister of war. This seemingly minor constitutional tweak in effect gave the army and navy a veto over the formation of any Japanese government. It was a tipping point that led Japan inexorably towards a military dictatorship.

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Barely a year later, Japan was at war again with China – this time for control of the whole country. The Mukden Incident, through which Japan had conquered Manchuria, had ended with the Tanggu Truce of 1933. The humiliating terms for China ceded not only Manchuria to Japan, but also control of the Great Wall and a 100-mile exclusion zone to its south. By 1937, gradual encroach­ments by Japan’s Kwantung (Manchurian) army had left Beijing all but surrounded.

On the 7 July 1937, an unplanned skirmish at the Marco Polo Bridge to the south of Beijing initiated all-out war between Japan and China. Major battles at Taiyuan and Shanghai were followed by the infamous Nanjing massacre, when as many as 300,000 men, women and children were murdered – a prelude to a decade of war and occupation that would cost more than 20 million Chinese lives. Aggressive international alliances were being forged. In November 1936, Japan had signed the Anti-Comintern Pact with Nazi Germany as the pair sought to contain Stalin’s burgeoning Soviet Union. The other eventual Axis power, Italy, would join a year later.

After 1936, the command economic model developed by the Kwantung army in Manchuria was increasingly deployed in Japan. Through the Industrial Bank of Japan, the government directed loans to the producers of war material. Further command-economy steps were taken via the National Mobilization Law of 1938, which downgraded domestic consumption: the focus was on guns not butter. It was an economic model that followed in lockstep with the policies of National Socialism in Germany. Japan’s ultranationalists, who dominated the army and navy, were now ready to embark on the creation of an economically self-sufficient ‘Greater East Asia Co-Prosperity Sphere’, aimed at curtailing western influence in the area. For Japan’s leaders, its imperial project was existential, driven by a Darwinian faith in the survival of the fittest. “It is clear as day,” the philosopher Kazunobu Kanokogi observed, “that if Japan fails to build an empire on the Asian continent, [as a nation] we are all doomed to destruction.”

A move to a total war footing was the natural next step. In addition to war with China, Japan’s fear of the Soviet Union led to the eruption of a full-scale border conflict, culminating in a heavy defeat for Japan at the battles of Khalkhin Gol, on the Mongolian-Manchurian border. But with the northern border neutralised by the signing of a pact with the Soviets in April 1941, Japan turned its attention southwards to complete the encirclement of forces loyal to Chiang Kai-shek, nationalist leader of the Republic of China, who were holed up in the remote western city of Chongqing. Looking to cut off Chiang’s sources of supply, Japanese troops occupied the north of French Indochina in September 1940.

The US: from isolation to intervention

By 1941, in a breathless decade of military conquest, Japan’s empire had expanded from an area of 245,000 square miles, including Korea and Taiwan, to 1.6 million square miles, covering Indochina (today’s Vietnam, Laos and Cambodia) and eastern China. The number of people living under Japanese rule had tripled from 100 million to 300 million.

While Japan was tearing up the geopolitical landscape of Asia, where was America? The United States had sent a naval expedition to open up trade with Japan in the 19th century, had fought a pitched battle on Chinese soil to defend its trading rights, had conquered the Hawaiian kingdom and seized the Philippines. But in the 1930s, the world’s undisputed economic superpower had gone missing.

After the First World War, the US had turned isolationist. The dominant narrative was that the Great War was a product of Europe’s corrupt, undem­ocratic monarchies. War profiteering was also blamed. Merchants of Death, a bestseller in 1934, was one of the many polemic publications that turned the American public towards neutrality in international affairs. The Great Dep­ression following the 1929 Wall Street Crash only height­ened the mood of introspection. The protectionist Smoot-Hawley Act (1930), which raised tariffs on over 20,000 imported goods, confirmed the US’s isolationist stance. Legen­dary columnist Walter Lippmann was expressing the majority view when he wrote in 1936: “The policy of the United States is to remain free and untangled.” In both 1932 and 1936, President Franklin D Roosevelt (FDR) ran on an isolationist ticket. Indeed Roosevelt, as commander-in-chief, presided over an armed force that was only the 18th largest in the world, with fewer soldiers than Belgium, Portugal and Switzerland.

Yet just a year after his second election victory, Roosevelt signalled a change of course. During a speech in Chicago on 5 October 1937, with Shanghai under siege and the most important US trade concession under threat, he warned: “The peace of the world is today being threatened… We are determined to keep out of the war, yet we cannot insure ourselves against the disastrous effects of war and the dangers of involvement.”

Where Roosevelt led, popular opinion followed. Every new Japanese action weakened America’s isolationist resolve: the ‘rape of Nanjing’, the invasion of Indochina, the strafing of the USS Panay on the Yangtse river. For the US public, however, it was probably the Tripartite Pact, signed by Germany, Japan and Italy in September 1940, that did most to alarm the court of public opinion. The American people, like Roosevelt, began to fear isolation in a totalitarian world. Increasingly, FDR initiated a covert defence of the free world. The Lend-Lease Act of March 1941, though primarily designed to offer military aid to Britain, also started to fund Chiang Kai-shek’s nationalist Kuomintang army’s resistance to Japan.

Most importantly, the hawks in Roosevelt’s administration finally managed to overcome his dovish secretary of state, Cordell Hull, who had hitherto resisted the call for meaningful sanctions against Japan. On 25 July 1941, Roosevelt froze all Japanese monetary assets held in the US, which immediately threatened Tokyo’s ability to supply its war machine. Japan’s cabinet board reported that “the empire will shortly be impoverished and unable to hold its own”. It predicted that the stocks of 8 out of 11 vital commodities would be depleted 50 per cent or more by 1942. Most significantly, Japan was unable to buy oil from Standard Oil of California, which had previously supplied some 80 per cent of its requirements.

The decision to attack

With a dwindling supply of petroleum, Japan faced the appalling prospect of having to give up its ambitions for a ‘Co-Prosperity Sphere’. In reaction to the US’s financial freeze and de facto oil embargo, on 3 September 1941, Prime Minister Konoe’s cabinet convened to discuss the ‘Outline Plan for the Execution of the Empire’s National Policy’, produced by Imperial General Headquarters, a council of top-ranking army and navy officers. Unless the western powers backed down, the cabinet resolved “… to go to war with the United States, Great Britain and the Netherlands if necessary”.

The accepted strategy for combating the west was for Japan to move rapidly to secure the oil-rich Dutch East Indies and the Philippines, a US colonial depen­d­ency. The supposedly formidable British military and naval garrison at Singapore would be taken, along with commodity-rich Malaya and Burma. Meanwhile, the main body of the Japanese navy would wait for the approach of the US fleet as it sailed to relieve the Philippines – which was indeed the proposal of ‘War Plan Orange’, as con­c­eived by the joint US Army and Navy Board in the 1920s. Here, at the Marshall Islands, in the western approaches of the Pacific Ocean, the Japanese navy would annihilate the US navy, just as the legendary Admiral Tōgō had decimated the Russian navy at the battle of Tsushima in the Russo-Japanese War in 1905.

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However, Admiral Isoroku ­­Yama­moto, who as commander-in-chief of its Combined Fleet held the pivotal role in Japan’s war strategy against the United States, had other ideas. He turned conventional wisdom on its head by planning a surprise attack on the US’s key Pacific naval base, located in Hawaii. By sinking America’s Pacific Fleet in a surprise attack – particularly its flotilla of aircraft carriers, which Yamamoto had identified as the key sea weapons of the coming war – he would seek to delay a US naval advance. This would give time for Japan to build up defences in the Pacific islands, and secure its resource supply lines within its newly acquired south-east Asian empire. At best, Yamamoto surmised that, after the destruction of the US navy at Pearl Harbor, Washington might even offer a truce.

Admiral Isoroku ­­Yama­moto turned conventional wisdom on its head by planning a surprise attack on the US’s key Pacific naval base

It was a risky strategy, opposed in Tokyo because the plan exposed Japan’s six main fleet carriers to the possibility of discovery and destruction. Furthermore, the aircraft carrier was a barely tested weapons system, with only Britain’s attack on the Italian navy at the battle of Taranto as an example of a carrier engag­ement. Yet Yamamoto was con­f­ident in the capabilities of Japan’s world-class torpedo planes, and indeed the torpedoes themselves, which had been designed in great secrecy in the interwar years to offer unparalleled speed, range and accuracy. Faced with continued opposition from his colleagues in Tokyo, Yamamoto nudged the decision his way by threatening to resign.

Final talks falter

As tensions rose with the US, on 27 November 1940 Japan sent former foreign minister Admiral Kichisaburō Nomura to Washington as ambassador. He was tasked with negotiating a lasting peace. In line with the Stimson Doctrine, the US demanded ‘open-door’ trade, with no regime change in Asia except through peaceful means and no interference in the affairs of other nations. These already tough conditions were stiffened further with American insistence, after the Tripartite Pact, that Japan should break ties with Adolf Hitler. The chances of a diplomatic breakthrough became even more remote in July 1941, when Japan signed the ‘Protocol Concerning Joint Defence and Joint Military Cooperation’ with France’s collaborationist Vichy government, which effectively ceded control of French Indochina to Tokyo.

Japan’s suspicion was that the US was playing for time while Roosevelt, having loosened Congress’s purse strings, set about rebuilding US military capability. And all the while, Japan’s stockpiles of raw materials – particularly oil – were running down. With the Japanese cabinet demanding the US and its western allies back down on its assets freeze, the scope for compromise was limited. When Emperor Hirohito’s counsellors advised him to choose the bellicose General Hideki Tojo as prime minister on 17 October 1941, the path to peace became vanishingly narrow. Tojo was an ultranationalist, who had asserted in an essay published in 1934 that Japan must “spread [its own] moral principles to the world, [for] the cultural and ideological warfare of the ‘Imperial Way’ is about to begin”.

At the imperial conference on 5 November, Hirohito approved Yamamoto’s plan of attack. The following day, Ambassador Nomura presented Washington with final concessions, known as Proposal A, for a partial withdrawal of Japanese troops from China. The US rejected this offer, having learned from their codebreaking intercepts that another proposal would follow. On the 20 November, Japan’s Proposal B offered withdrawal from southern Indochina if the US would unfreeze Japan’s assets and refrain from supplying Chiang Kai-shek’s armies in China. Both proposals were declined. Aware from intercepts on 26 November that Japan would launch an attack sometime after 29 November, Roosevelt knew that war was all but inevitable. From detected troop movements, the assumption was that the target would be somewhere in south-east Asia, though there were uncertainties about the whereabouts of Yamamoto’s Combined Fleet. In fact, it was hiding under radio silence in the remote Kuril Islands, at the northernmost tip of Japan.

In the meantime, with nothing further to gain from negotiations, Cordell Hull, the US secretary of state, presented Ambassador Nomura with a 10-point ultimatum, including the demand that Japan withdraw from all of China and Indochina. Faced with utter defeat and humiliation if he accepted the American terms, Hirohito, at a conference with General Tojo on 1 December 1941, gave the final sanction for simultaneous attacks on Pearl Harbor, the Philippines and British Malaya. Japan’s invasion of China, which had started with the Mukden Incident in 1931, had become the casus belli that had launched Japan into war with the world greatest empire, Britain, and the world’s most powerful nation: the United States.

This article was taken from the BBC Collector's Edition Pearl Harbor bookazine, first published in 2019