Why did Japan attack Pearl Harbor? The reasons, explained

Why did Japan attack Pearl Harbor? Japan's leaders, at war in China since 1937 and cut off from American oil in 1941, decided to seize the oil and raw materials of Southeast Asia by force. The US Pacific Fleet at Pearl Harbor, Hawaii, was the one force that could stop them, so on 7 December 1941 they struck it first, hoping to win time to build a defensive empire. Instead the attack brought the United States into World War 2.

In short

• Japan's war of conquest in China led the United States to cut off its oil in 1941, leaving Japan to choose between retreat and war.
• Japan's leaders chose war and planned a surprise blow to knock out the US fleet while they took Southeast Asia.
• The attack killed 2,403 Americans and united a divided country behind entering the war, the opposite of what Japan needed.

What led Japan to war with the United States?

The road began in China. In 1931 the Japanese army seized Manchuria, in China's northeast, and in July 1937 a clash at the Marco Polo Bridge near Beijing grew into full-scale war, as the State Department's historians describe. Japan's army won battles and took China's coast and cities, including Nanjing, where its troops massacred civilians, but it could not end the war. China fought on from the interior, supplied in part by Britain and the United States.

Japan's leaders, increasingly dominated by the army, believed the country needed its own sphere of resources in Asia to survive as a great power. Japan had little oil, rubber or tin of its own. The oil it ran on came mostly from the United States, and the nearest alternative was the Dutch East Indies, today's Indonesia.

Why did the oil embargo matter so much?

In September 1940 Japan signed the Tripartite Pact with Germany and Italy and moved troops into northern French Indochina, now Vietnam. In July 1941 it occupied the south of Indochina as well, within striking distance of British Malaya and the Dutch oil fields. The United States answered by freezing Japanese assets, which in practice cut off its oil exports to Japan, and Britain and the Netherlands followed.

That turned a slow war into a deadline. Without imported oil, Japan's navy and army would run down their reserves within a couple of years, and every month of waiting made them weaker. The United States offered to lift the embargo only if Japan withdrew from China and Indochina, which Japan's military leaders refused. On 26 November 1941 Secretary of State Cordell Hull restated that demand. On 1 December a conference in the emperor's presence approved war.

Date	Step toward war
1931	Japan seizes Manchuria
July 1937	Full-scale war with China begins
September 1940	Tripartite Pact with Germany and Italy; troops into northern Indochina
July 1941	Japan occupies southern Indochina; the US freezes Japanese assets and halts oil exports
26 November 1941	Hull restates US terms; the Japanese strike force sails from the Kuril Islands
1 December 1941	Imperial conference approves war
7 December 1941	Attack on Pearl Harbor, and across Southeast Asia within hours

Why attack Pearl Harbor in particular?

Japan's real target was Southeast Asia: Malaya, Singapore, the Philippines and the oil of the Dutch East Indies. The US Pacific Fleet, moved to Pearl Harbor in 1940 as a warning, sat on the flank of that advance. Admiral Isoroku Yamamoto, commander of Japan's Combined Fleet, argued that the only chance against a country with many times Japan's industrial power was to destroy the fleet at the start. That would buy months to take the southern resources and build a defensive ring of islands. Japan's leaders hoped that the United States, facing a long war to win it all back, would accept a negotiated peace.

The plan was a gamble, and some of Japan's own planners said so. Yamamoto himself, who had lived in the United States, warned that Japan could not win a long war.

What happened on 7 December 1941?

Six Japanese aircraft carriers crossed the North Pacific in radio silence. Just before 8 a.m. on Sunday, 7 December, the first of two waves of aircraft, 353 in all, struck the fleet at anchor and the airfields around it. In under two hours they sank or damaged more than 20 ships, including all eight battleships present, and destroyed 188 American aircraft. The Naval History and Heritage Command gives the toll:

	Killed or missing	Wounded
Navy	2,008	710
Marine Corps	109	69
Army	218	364
Civilians	68	35
Total	2,403	1,178

Almost half of the dead, 1,177, were aboard the battleship Arizona. Japan lost fewer than 100 men and 29 planes.

Tactically it was a stunning success. Strategically it missed what mattered. The fleet's aircraft carriers were away at sea, and the attackers left the oil tanks and repair yards untouched. All but three of the damaged ships returned to service. Within six months, at Midway, American carriers sank four of the six Japanese carriers that had attacked Pearl Harbor.

How did it bring the United States into the war?

Before Pearl Harbor, Americans were divided over joining the war, and a strong movement wanted to stay out. Japan's note breaking off negotiations, meant to be delivered just before the attack, reached Hull only after it had begun, so the attack looked like a betrayal as well as a surprise. On 8 December President Franklin Roosevelt asked Congress to declare war, calling 7 December "a date which will live in infamy". The Senate voted unanimously; in the House only one member, Jeannette Rankin, voted no, the National Archives records. On 11 December Germany and Italy declared war on the United States, and the regional wars became one world war, one that ended in the Pacific with the atomic bombs on Japan.

At home the shock had a darker side. In February 1942 Roosevelt signed an order that led to the forced removal of about 120,000 people of Japanese ancestry from the West Coast to camps, most of them US citizens.

Did Roosevelt know the attack was coming?

No. The theory that Roosevelt knew and let it happen, to draw the country into war, has circulated since 1941, but the evidence does not support it.

Claim	What the evidence shows
US codebreakers were reading Japan's messages	They read diplomatic messages, which showed war was near but never named Pearl Harbor; the navy's operational code was barely broken
The strike force's radio signals were picked up	Japanese records say it kept strict radio silence, and no reliable intercept located it
The carriers were moved away to save them	Two were delivering aircraft to Wake and Midway and the third was in California; a planner sacrificing the fleet would hardly keep the most valuable ships out of a trap he set
Washington warned no one	A war warning went to Pacific commanders on 27 November; it expected an attack in Southeast Asia, not Hawaii

Ten official investigations, from 1941 to 1995, found failures of command and intelligence and an underestimation of what Japan could do, but no plot, according to the summary of the theory and its rebuttals. The historian Roberta Wohlstetter described the real problem as signals buried in noise: the warnings existed, scattered among many false alarms. That pattern, of clues visible only afterward, has been found again in later surprise attacks, including the attacks of 11 September 2001, and telling hindsight from foresight is part of how historians judge the past.

Check yourself

• Without looking back: why did the US oil embargo put Japan on a deadline, and what did the United States want in return for lifting it?
• What was the attack on Pearl Harbor meant to achieve, and what did it fail to destroy?
• Why do historians reject the claim that Roosevelt knew the attack was coming?

Sources

• Japan, China, the United States and the road to Pearl Harbor, 1937–41 — Office of the Historian, U.S. Department of State
• Pearl Harbor attack, 7 December 1941 — Naval History and Heritage Command, U.S. Navy
• "A date which will live in infamy" — U.S. National Archives
• Pearl Harbor advance-knowledge conspiracy theory — Wikipedia