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The Forgotten Reason Japan Attacked America At Pearl Harbor

Imperial Japan Pearl Harbor

The day after roughly 350 Japanese warplanes came screaming down over Pearl Harbor and sank or crippled eight of the U.S. Pacific Fleet’s battleships over a span of ninety minutes, Japanese newspapers published a statement by Emperor Hirohito declaring war with the United States and the United Kingdom and outlining its rationale for the attack.

“It has been truly unavoidable and far from Our wishes that Our Empire has been brought to cross swords with America and Britain… They have obstructed by every means Our peaceful commerce and finally resorted to a direct severance of economic relations, menacing gravely the existence of Our Empire. Patiently have We waited and long have We endured, in the hope that Our government might retrieve the situation in peace. But Our adversaries, showing not the least spirit of conciliation, have unduly delayed a settlement; and in the meantime they have intensified the economic and political pressure to compel thereby Our Empire to submission.”

Why exactly did Japan elect to attack a country with twice the population, five times the steel production and seventeen times the gross national income? The answer all came down to a U.S. embargo on imposed in response to Japan’s brutal invasion of China.

The fateful collision course between the United States and Japan was set ninety years earlier when in 1853 Commodore Matthew Perry arrived in a feudal, isolationist Japan and demanded it open itself to foreign trade. In the ensuing Meiji Restoration, the Japanese Emperor seized power from the feudal shogunate and implemented a policy of rapid modernization to avoid being exploited by Western imperialists as happened to China and India.

In practice, this meant Japan not only industrialized and modernized its armed forces, but also sought its share of colonial spoils. In 1894-95, the Japanese fleet defeated a beleaguered China, then effectively seized and colonized the “Hermit Kingdom” of Korea. Then in 1904-1905, the Imperial Japanese Navy defeated Tsarist Russia in the first major victory by an Asian country against a European power in modern times. Finally, early in World War I Japanese forces captured Germany’s colonial possession in China on behalf of the Franco-British Entente alliance. Tokyo was rewarded with permanent control of those territories post-war—much to the anger of the Chinese, who had also supported the Entente’s war effort.

The Japanese military had its sights set on grabbing more Chinese territory. Using the pretext of an attack on a Japanese railroad in Mukden (modern-day Shenyang) actually staged by a nationalistic Japanese officer, Japanese troops seized Manchuria, assaulted the Great Wall of China in 1933, and created the puppet state of Manchukuo. Then the absurd Marco Polo Bridge incident in 1937 was used to justify a wider Japanese invasion in 1937 that seized most of China’s wealthy coastal cities.

Japan’s civilian government was not universally supportive of the military’s aggressive campaigns. However, several anti-war politicians were brazenly assassinated by Japanese officers, and the civilian government came to be completely marginalized by the military.

Tokyo eventually dubbed its rapidly growing empire the Asian Co-Prosperity Sphere. The Sphere was intended to rally nationalists across Asia opposing the depredations of Western imperialism. However, while the Sphere did attract some supporters such as Thai Prime Minister Plaek Phibunsongkhram, in practice the Co-Prosperity Sphere simply meant exchanging one form of foreign exploitation for another.

But the Japanese invasion of China caused relations with Washington to deteriorate. The United States was not entirely innocent of Asian colonialism—it too profited from the increased trade enabled by the Opium Wars, deployed soldiers to suppress the Boxer Rebellion, and effectively occupied the Philippines between 1898 and 1935. However, Washington had cultivated ties with the Chinese Nationalists and was alarmed by Japan’s military buildup. Moreover, America was ill-inclined towards Tokyo due to increasing reports of Japanese atrocities which included massacres such as the rape of Nanjing and the operational testing of biological weapons on Chinese civilians.

Finally, after Japanese forces invaded French Indochina (modern Vietnam and Cambodia) in June 1941, President Roosevelt implemented a ban on iron, steel and oil exports to Japan jointly with Australia and the United Kingdom.

Japan’s Pacific-spanning Empire depended on oil—and Japan had been importing 80 percent of it from the United States. Tokyo did have fifty-three million gallons of oil in reserve—a supply which could sustain its Empire for roughly a year.

Japan would have to withdraw from its Empire unless it could get more oil. There was a convenient supply of oil nearby at hand in the Dutch colonies of the East Indies. As the United Kingdom was tied down fighting Nazi Germany and the Netherlands was already occupied, Japan forces could likely use their existing oil reserve to capture the vital oil wells—but with the likely consequence of drawing the United States into war.

Thus, Tokyo began looking into a military option to cripple the U.S. military long enough to secure the Dutch oil wells and present a defeated U.S. public with a fait accompli.

Admiral Yamamoto warned Japanese militarists that he could only guarantee six months of victories—but he dutifully went ahead and planned the Pearl Harbor attack, which by any conventional military standard was an extraordinary success. Japan’s simultaneous invasions of the Dutch East Indies, Singapore, Burma, Hong Kong, Malaya and the Philippines likewise initially succeeded spectacularly.

But if the Pacific War were a board game, Japan had set up its pieces to pull off a series of killer movies early in the first few turns—but was still playing with a lot fewer pieces. Moreover, the Pearl Harbor raid failed to remove the most important U.S. pieces—its force of aircraft carriers, each carrying a hundred fighters and bombers that could attack targets hundreds of miles away.

After six months of Japanese victories, U.S. carriers sank four Japanese carriers in the Battle of Midway in June 1942—and subsequently embarked on an “island hopping” campaign that relentlessly dismantled the Japanese Empire over the next three years.

Undeniably, the U.S. oil embargo had backed Imperial Japan into a corner. The Japanese military’s instincts, supported by a succession of victorious conflicts against numerically superior foes, was to strike out. Tokyo trusted in the valor and prowess of its soldiers and the incompetence of its enemies rather than deciding to back down before a superior correlation of forces.

It follows that diplomatic compromises between Washington and Tokyo could have averted the Pacific War—but only with the consequence of enabling Japan’s brutal occupation of China, which is estimated to have caused the death of three to five million Chinese citizens.

Why Japan Attacked Pearl Harbor

Sébastien Roblin holds a master’s degree in conflict resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.

Written By

Sebastien Roblin writes on the technical, historical, and political aspects of international security and conflict for publications including the 19FortyFive, The National Interest, NBC News,, and War is Boring. He holds a Master’s degree from Georgetown University and served with the Peace Corps in China.  



  1. Steven Songstad

    July 10, 2021 at 5:01 pm

    How is this the “forgotten reason”? This is precisely what was taught in high school and subsequent college courses if one took classes in modern world history.

  2. Larry

    July 10, 2021 at 5:54 pm

    I also wonder what was the forgotten reason?
    This article just rehashes all the same info with a headline to induce you to read it. It’s a well written article just a bad headline.

  3. Mikie

    July 10, 2021 at 6:01 pm

    It’s why the article is titled “the forgotten reason”. Yes, it was taught but we often forget when push comes to shove an adversary picks two options strike or pull back. The Japanese chose the first.

  4. James

    July 10, 2021 at 6:05 pm

    Yes, I don’t think anyone has forgotten the atrocities the Japanese has inflicted on US and Asians during world war 2.

  5. Arup

    July 10, 2021 at 6:37 pm

    Very correct. That is what remember from history lessons. Just a title to attract attention.

  6. FastestFalcon

    July 10, 2021 at 9:02 pm

    I was kind of hoping the “forgotten reason” had something to do with anti-Japanese immigration laws passed by the USA during the decades before the war, as those soured relations and public sentiment between the two countries early on, but I guess the embargo is important information too, even if it’s not really “forgotten.”

  7. Steve Alberts

    July 10, 2021 at 9:42 pm

    I notice the article mentioned the US/British/Australian embargo on oil, with 80 percent coming from US. Did the duplicitous Japanese ever consider BUYING OIL from the Dutch East Indies, rather than stealing it? This is the type of treachery they were doing, going in and raping China, attacked the Philippines and US in 1941, and were set on dominating the Western Pacific by any means necessary. Most of the rice and food supplies were exported to Japan from the Phillipines causing millions to starve. I believe 1 in 7 Filipinos perished between 1941 and 1945. Add to this the abuse and maltreatment of Allied prisoners, the Bataan Death March, and the Japanese “Kill All” order for prisoners at the end of the war, Japan deserved what it got with the firebombing of Tokyo and the dual atomic attacks. Who wants to debate this?

  8. Ned Clark

    July 11, 2021 at 2:04 am

    Suggested edit/addition:

    “The answer all came down to a U.S. embargo on [fuel oil] imposed by the U.S., in response to Japan’s brutal invasion of China.”

  9. M.G. Jackson

    July 11, 2021 at 7:46 pm

    Not often these days you see IJG propaganda written so boldly. It’s more than a bit like making excuses for Hitler’s invasion of Poland, France and lots of other places. Of course that’s exactly it-excuses for invasion, plunder, rape and murder. The IJG was no more giving room to nationalist that they had done to Korea (occupied 1910) or Formosa.

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