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Why Did Japan Attack Pearl Harbor?

After more than 75 years, the question remains for students of the history of World War Two: Why did Japan attack Pearl Harbor?

The attack on Pearl Harbor ranks as the most successful military surprise attack in the early years of combined naval/aerial combat. On December 7, 1941, the Imperial Japanese Navy Air Service struck the United States naval base at Pearl Harbor, Hawaii Territory.  The Attack on Pearl Harbor on December 7, 1941, directly caused America’s entry into WW2 which led to the eventual launch of the atomic bombs on Hiroshima and Nagasaki, an outcome that spelled disaster for the Japanese. The Japanese managed to damage almost 20 U.S. naval vessels, of which 8 large battleships, 200 airplanes, and killing over 2,000 Americans, but why did the Japanese attack America in the first place? And what were they trying to achieve by attacking Pearl Harbor, specifically?

Why Did Japan Attack Pearl Harbor?

The U.S. and Japan had been butting heads for decades and it was inevitable that things would eventually culminate into a war. Japan had imperial ambitions to expand to China to solve some demographical and economical problems and to take over the Chinese import market. When in 1937 Japan decided to declare war on China, America was very against this aggression and responded with trade embargoes and economic sanctions. Specifically, the oil embargo that America organized with the British and the Dutch was a thorn in the side for Japan, which imported 90% of its oil. Without oil Japan’s military could not function and all war efforts would come to an end. Negotiations had been going on for months between Washington and Tokyo, without any resolution, so Japan decided to attack first.

Why Attack Pearl Harbor?

As war was inevitable, Japan’s only chance was the element of surprise and to destroy America’s navy as quickly as possible. Japan wanted to move into the Dutch East Indies and Malaya to conquer territories that could provide important natural resources such as oil and rubber. By destroying a large portion of the American fleet, they hoped to conquer the Philippines and Malaya while America was still recovering from its own damages – simultaneous attacks were launched on these places while Pearl Harbor was taking place.


Let’s return to our main question: why did Japan attack Pearl Harbor? Ultimately, Japan hoped that America would accept defeat and that Japan could create a fortress that would stretch across the whole Pacific Rim.

Roosevelt expected an attack by the Japanese, but conspiracy theories claiming that he knew that they were going to strike Pearl Harbor have been rejected by most scholars. The Government rather expected Japan to attack American targets in Thailand or the Dutch East Indies than a target this close to home. The Chicago Tribune published a top-secret war plan, “Rainbow Five” on December 4, 1941, in which the War Department made preparation arrangements for war with Japan.

Intelligence Warnings of the Attack Before Dec. 7, 1941

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The day before the death of Sara Roosevelt, Franklin Roosevelt’s mother, the State Department’s rebuff of Japanese Prime Minister Konoye’s urgent request for a private talk with Roosevelt convinced the Japanese to begin serious plans for an attack.

At a cabinet meeting on September 6, 1941, Admiral Isoroku Yamamoto was told to attack unless Konoye somehow achieved peace terms with the United States that would not spark a revolution at home, an uprising in Korea, or the restoration of Chinese morale. Hirohito had been shot at twice, once by a Japanese communist, once by a Korean nationalist. The better men of two cabinets had been murdered or wounded because they were seen as too accommodating to the foreigners who wanted to colonize Japan or reduce the nation that had never lost a war in modern times to a vulnerable third-rate power. Konoye himself had been threatened with assassination if he made too many concessions, and there had been serious attempts to overthrow the emperor in favor of his brother or his son. Hirohito knew that his dynasty itself could be wiped out like the Romanovs or marginalized, as the Japanese themselves had done to the Korean royalty if he bowed to demands that the Japanese saw as not merely insulting but insane.

Yamamoto, who spoke fluent English, had studied at Harvard, and in happier times had hitchhiked across the United States, knew that Japan could not conquer, or even defeat, the United States. The Japanese grand strategy, if war could not be avoided, was to inflict enough damage and seize enough territory that the Americans would guarantee Japanese sovereignty in return for an armistice and restoration of all or most of what Japan had taken outside Korea and perhaps Manchuria.

Theoretical plans for a Japanese attack on Pearl Harbor had existed for decades. General Billy Mitchell had warned as early as 1924 that the next war would be fought with aircraft carriers. The U.S. Navy’s Admiral Harry Yarnell conducted a simulated attack by carrier-based aircraft in 1932 as part of a war game. The Navy judges ruled that it would have sustained substantial damage if the attack had been genuine, and the attackers won the war game.


Yamamoto had delivered his updated contingency plan for an attack on Pearl Harbor on January 7, 1941, less than a month after the British aerial torpedo attack on Taranto. Minoru Genda, Japan’s genius of planning, called Yamamoto’s initial plan “difficult but not impossible.” More information was needed. By the summer of 1941, Korean patriots who kept an ear to the wall at the Japanese consulate in Honolulu through Korean servants and loyal Japanese-Americans were picking up rumors of intense Japanese interest in the depth of water in the harbor and the strengths and weaknesses of the Army and Navy installations in Hawaii.

Roosevelt’s restriction on Japan’s oil supply shifted Japanese planning into high gear. War was now the only alternative to economic strangulation and political revolution.

In the final months leading up to the attack, the U.S. government issued a memorandum stating, “The Japanese government does not desire or intend or expect to have forthwith armed conflict with the United States. . . . Were it a matter of placing bets, the undersigned would give odds of five to one that Japan and the United States will not be at ‘war’ on or before March 1 (a date more than 90 days from now, and after the period during which it has been estimated by our strategists that it would be to our advantage for us to have ‘time’ for further preparation and disposals).”

On December 1, 1941, the emperor met with his privy council. “It is now clear that Japan’s claims cannot be attained through diplomatic means,” Tojo said. The emperor—perhaps more gun-shy than the elder statesmen—asked for a vote. The cabinet voted unanimously for war. Hirohito agreed. The Japanese fleet was told to attack Pearl Harbor on December 7 unless it received a last-minute cancellation because of a sudden change in America’s attitude. Kurusu and Nomura—who had been sincere in seeking peace until they received the Hull note—were told to stall for time. Tojo summed up the situation: Japan, the one Asian, African, or South American nation that had modernized instead of being colonized, could not accept the American demands without riots at home, revolt in Korea, and reversal in Manchuria. “At this moment,” he declared, “our Empire stands on the threshold of glory or oblivion.”

This article is part of our larger selection of posts about the Pearl Harbor attack. To learn more, click here for our comprehensive guide to Pearl Harbor.


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"Why Did Japan Attack Pearl Harbor? A Comprehensive Analysis" History on the Net
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