Why did Japan attack Pearl Harbor?

I know that the USA had placed an oil embargo on Japan and without this oil, Japan’s economy would soon grind to a halt. But how did attacking Pearl Harbor help the Japanese out? From what I’ve read they knew if the Pacific War lasted more than a year or two, they would not be able to win. That’s a mighty big gamble to take. Why did they not just ignore the USA and start attacking resource rich areas in SE Asia? What would bringing the USA into the war help? Would the USA had done anything if Japan had started attacking Indonesia and SE Asia but did not touch US held areas such as the Phillipines considering the strong isolationist attitude at the time?

They could of only had been able to take so much territory by assuring the most powerful naval fleet in the area had been knocked out, and when this was completed, the Japanese in a position of strength expected negotiations to be in their favour when initiated by the US government. They expected too much, no matter how isolationist, they would of come round eventually if attacked like that.

Japan invaded North China in 1937. It turned southward in 1939, taking over Hainan Island and the Spratlys: coral islands offering potential havens for planes and small naval craft. By 1941, Japan (after joining the Tripartite Pact on September 27, 1940) poised for further expansionist adventures into SE Asia: Malya, the Philippines, and the Netherlands East Indies. Japan was convinced that necessity and self-protection demanded they take over the vast resources of those lands to break through encirclement and beat off gthe challenge of any one or a combination of their international rivals: USA, Great Britain, and USSR.

In essence, China was the touchstone of Japanese-American relations, but only part of the “Greater East Asia Co-Prosperity Sphere,” a concept which made the democracies uneasy. The fulfillment of this Sphere, incorporating as much as Japan could, was imperative to the Japanese. Japan had a long list of grievances against the USA, the foremost being the recognition of Chiang Kaishek’s regime and the non-recognition of Manchukuo. USA and European presence in Asia was a constant irritation, and the Japanese press assured such intruders that Japan would slam the Open Door in their faces.

In addition, Britain was at war with Japan’s allies (Germany and Italy), so what helped the British hindered the Axis. Also, Japan considered that Washington’s bolstering of London perpetuated the remnants of British colonialism and hence the obnoxious presence of European flags on Asian soil.

Japanese anger also focused on the embargoes which the US had slapped on American exports to Japan, and by the end of 1940, Washington had cut it off from all vital war materiels except petroleum. Tokyo also had an old bone to pick with Washington: the immigration policy which excluded Japanese from American shores and refused citizenship to those Japanese residents not actually born in America. Above all, it considered America’s huge naval expansion program aimed directly at it. Since stationing of a large segment of the Fleet at Pearl Harbor in the spring of 1940, the US Navy had stood athwart Japan’s path: a navy which Japanese admirals thought capable of menacing the very existence of Japan. To the Japanese, we were the “Cancer of the Pacific.”

[source: At Dawn We Slept, The Untold Story of Pearl Harbor, Gordon W. Prange (1961)]

Is there any truth to this? What was America’s plan with the Pacific fleet? Was it meant just as a deterent or as a means of projecting its power? Did America actually have colonial ambitions over Japan and Asia (besides the Phillipines and several islands in the Pacific)?

We did not have any aggressive policy. “With all our desire to keep American out of war and at peace with all nations, especially with Japan, it would be the height of folly to allow ourselves to be lulled into a feeling of false security,” Ambassador Grew wrote on January 1, 1941, in his diary.

Japan’s Foreign Minister, Yosuke Matsuoka, said in a speech in Tokyo in January, 1941: “The Co-Prosperity Sphere in the Far East is based on the spirit of Hakko Ichiu, or the Eight Corners of the Universe under One Roof… We must control the western Pacific.”

Admiral Koshiro Oikawa became navy minister on September 4, 1940. He thought Japan might be “running some risk of picking Germany’s chestnuts out of the fire,” but he believed that “America was so unlikely to go to war that the situation was fairly safe.” Even though he preferred diplomatic and naval pressure to military action, before the end of January 1941, he assured his countrymen that “the navy is prepared fully for the worst and… measures are being taken to cope with the United States naval expansion.” (By that time his head bulged with the weightiest of secrets. He knew a lot more than he was prepared to tell. Nor did he dare tell all he knew.)

As the other posters have said before, Japan seemed to consider war with the U.S. inevitable. After all, the U.S. was actively aiding the Chinese and British, and held a lot of Pacific islands that were right in the heart of the Japanese Pacific Co-Prosperity Sphere. I suppose their thinking was a lot like that of the Germans in regards to Russia. “We have to fight them eventually, so we might as well do it now when they’re not expecting it.”

The Japanese were certainly hoping that Pearl Habor would be so damaging to American morale that it would make a war unnecessary. They all knew that Japan could not hope to match the manufacturing and shipbuilding capacity of the U.S. They were probably counting on America’s isolationism and a resounding military victory to force the U.S. to cede its Pacific territories without any further fighting.

Of course, that’s not how things worked out.

The Japanese knew that attacking Pearl Harbor was a gamble, but in retrospect it was nothing but a huge mistake. It is likely but not certain that the US would have intervened had Japan moved into Southeast Asia (disregarding the Philippines for the moment), but had we done so, we probably would have been defeated. Although the US had more total battleships in 1941, theirs were newer and they had superior gunnery skills and torpedos, and could put more and better aircraft into the air. Furthermore, the absense of the galvanizing effect of Pearl Harbor would have meant less resources and public support for the war effort. In a sense, Vietnam would have come 25 years early.

The Japanese knew that attacking Pearl Harbor was a gamble, but in retrospect it was nothing but a huge mistake. It is likely but not certain that the US would have intervened had Japan moved into Southeast Asia (disregarding the Philippines for the moment), but had we done so, we probably would have been defeated. Although the US had more total battleships in 1941, theirs were newer and they had superior gunnery skills and torpedos, and could put more and better aircraft into the air. Furthermore, the absense of the galvanizing effect of Pearl Harbor would have meant less resources and public support for the war effort. In a sense, Vietnam would have come 25 years early.

That’s just not the case. War with the Allies was obviously a terrible mistake; the specific tactical decision to attack Pearl Harbor was a brilliant and audacious move. The entire initial Japanese offensive ranks among the greatest works of military genius in the history of warfare.

The inevitability of war with the United States is being hugely underestimated here; both sides were actively preparing for war and had pretty much assumed that war would commence at any moment. The United States and Germany were already shooting at each other in the Atlantic; it’s absurd to pretend that the attitude between the USA and the Axis was anything short of outright hostility. If the USA didn’t enter the war on December 7, 1941, it absolutely would have in a matter of a few months.

Based on that reality, the attack on Pearl certainly didn’t HURT the Japanese cause any; they lost just 29 planes in the attack, and the Allies were so thoroughly crushed in the western Pacific that the IJN lost no time or positioning having the fleet that far out. The position of Japan when the USA began its counterattack in spring 1942 was no worse than it would have been had they stayed away from Pearl Harbor, even if you give them a few extra months of prep time. In any event, fortifying islands didn’t work.

I can’t see why the U.S. would have been defeated had Pearl Harbor NOT been attacked. What did the U.S. gain by having Arizona blow up? A galvanized public? Bah; almost everything thrown at the Japanese in 1942 already existed.

As RickJay notes, the U.S. was also planning to have a war with Japan. (U.S. planners were not as brilliant as Yamamoto, so there is no reason to believe that the U.S. plan–luring the Japanese fleet into a big gun shootout near the Philipines–would have worked, but the U.S. military was certainly expecting the war to come.)

Cite, please, for the US planning a war with Japan.

Yamamoto was actually against the attack, initially. On January 26, 1941, Yamamoto wrote a sarcastic letter to Ryoichi Sasakaw, an ultranationalist, which actually warned the far right bluntly that the US was not, as certain wishful thinkers believed, a hollow giant to fall and smash to pieces at the first blow. For Japan to conquer the Americans, it must land its forces on the Pacific coast, push across mountains, deserts and vast plains, fighting every inch of the way, and occupy Washington itself. The sarcasm was lost when Japan’s nationalists deliberately distorted Yamamoto’s meaning. They published an altered version of the statement and created the impression that Yamamoto promised to dictate peace in the White House. Yamamoto could not create dissension by contradicting it publicly.

[Prange, op.cit., page 11]

Ibid, page 170]

The Japanese did think that the US meant to follow up the embargo with a declaration of war, and responded not with surrender but with further belligerence.

Ibid page 171]

I have studied this issue carefully a long time, and I would side with those who say that while the US saw war with Japan as an increasing likelyhood throughout 1941, there was no US plan to attack Japan or Japanese interests.

The best evidence for this is the complete lack of war preparations – offensive or defensive – in the Philippines and Hawaii. When there was belief that an attack was imminent, the US forces were so completely unprepared for war that warnings were of no avail.

The main Japanese aim was the Philippines and (nowadays) Indonesia, and Hawaii was attacked to ward off an American counter-attack by destroying the Pacific fleet.

As has been noted, Japanese regard for American fighting spirit was low. They considered Americans to be “effiminate cowards” who would run from a fight as soon as they were delivered a stunning blow. Hitler thought the same thing (as did OBL, by the way).

Two really good books that detail the development of Japanese thought on the run up to war, including all the political infighting in Japan as the “war party” eventually took over Japanese politics are:

“The Rising Sun: The Decline and Fall of the Japanese Empire, 1936-1945”, written by John Toland.

Edwin Hoyt’s book “Japan’s War – The Great Pacific Conflict 1863 to 1952”
Edwin Palmer Hoyt

both are told from the Japanese point of view, with lots of resources from Japanese politicians, military figures and Japanese newspapers, government records, etc.

this website has many of the good books listed:


The Arizona is the key. After Pearl Harbor, the only weapons the US had for immediate deployment against the Japanese were aircraft carriers and submarines. If we had gone to war with battleships, and the mentality of battleship admirals, our navy would have ended up like the Repulse and the Prince of Wales, ie sunk with nothing to show for it. The Japanese attack forced us to use the weapons that in the end proved to be the most effective. The combination of carrier assaults and the strangling of the Japanese logistical lines by subs made the outcome of the war inevitable.

As Yamamoto observed - “I fear we have awakened a sleeping giant.”

The U.S. had seen Japan as an inevitable enemy as early as the 1904-1905 war between Japan and Russia. Among the various “color” plans laid out to prepare for war with Germany, Russia, The U.K.(!), Spain, and others, the War Plan Orange, focusing on Japan, was the most comprehensive and detailed. It was originally laid out in 1911 and updated several times, thereafter. Inthe 1930s, when the U.S. began to see the possibility of war with multiple countries, it developed War Plan Rainbow that actually provided an early blueprint for the war we waged from 1942 through 1945.


As has been noted in most works on Pearl Harbor, the Japanese air attack very closely followed the war games performed by the U.S. Navy and Army Air Corps beginning around 1927.

The lack of preparation was a result of the decreased military spending in an isolationist atmosphere in the midst of a depression rather than a conscious decision by the military to pretend that there would be no war with Japan. War Plan Orange was actually used to guide the actions of the Army forces in early WWII–the Philipines was lost following the underfunded version of War Plan Orange-3 that was then in effect.

The U.S. military thought that war with Japan was inevitable. They may have initially taken the wrong approach in how to win it and they may have lacked the funds to assemble the matériel to carry out their plan, but very definitely “the U.S. military was certainly expecting the war to come.”

(I’m not sure what the Yamamoto reference has to U.S. preparation. I am familiar with his letter declaring the need to get a surrender on the steps of the capital, of course, having quoted it on this board.)

In 1939, the japanese Army Staff made up two war plans…NWR was the codename for an attack on Russian Siberia, SWR was the plan for the invasion of the Dutch east Indies, French Indochina, the Philippine Islands, etc. Th reason that the Japanese chose the second course: the Japanese war machine needed oil…and these was very little in Siberia, but lots in the east indies.
Ironically, the USA caused the japanese to turn awy from Siberia, when President Roosevelt embargoed shipments of California-produced oil to japan!
I’ve ofetn wondered how WWII might have turned out…a Japanese attack upon Siberia might have resulted in the Germans beating Russia!
However, the Japanese military realized that Japan had resources sufficient for a relatively short war. Indeed, once they lost most of their aircraft carriers at Midway Island, the end was in sight for japan. Of course, the japanese missed an excellent chance at Pearl Harbor…had they invaded Hawaii after the air raid, it is hard to see how the US could have won the war!

Actually, that quote is apocryphal.

Again, this doesn’t meet a common sense examination. The U.S. Navy would not have sent battleships in alone; they would have fought alongside carrier task groups, which is precisely what both they and the Japanese did for pretty much the entire war. The JAPANESE still had a large number of battleships after Pearl Harbor, but having them sunk Singapore-style was not the reason they lost the war. As you will note, at Midway, the American naval aircraft went straight for the Japanese carriers, not the battleships. Once the carriers were sunk, the battleships fled. I am sure the Americans would have done the same. .

Logically, if the loss of battleships was irrelevant at Pearl, it would have been irrelevant had they been sunk in the middle of the Pacific. Likelier still, the battleships would not have even been targeted at Midway.

The U.S. Navy was NOT strengthened by the Pearl Harbor disaster; it’s apologism to say that it wasn’t the catastrophe that it was. The followup successes - the tie in the Coral Sea followed by the amazing victory at Midway - would a product of skill, audacity and luck, not the loss of battleships.

Indeed, while it’s true ultimate victory was a product of a colossal American advantage in industrial capacity, let’s not forget that the U.S. in 1942 beat back Japan with a force that was in many ways inferior. The Japanese plan, tactically, was just fine; the U.S. comeback was phenomenal and you could never have expected it to happen that fast. It was a remarkably brave and brilliant performance, and owes a lot to the genius of Nimitz and his admirals and the ferocity and skill of American servicemen. I’ve always been impressed that the U.S. could even launch attacks as far foward as they did as early as they did.

Puh-lease. Pearl Harbor was just a port. A big and important one, certainly, but its loss wold have had zero impact on U.S. manufacturing capability. All the loss of Hawaii would have accomplished is buying the Japanese the few months it took the U.S. to retake the islands and rebuild the port. Given how remote Hawaii is from Japan, it would be extremely difficult for them to resupply it during any kind of assault. If they did keep their navy hanging around Hawaii to defend it, that would make the conquest of their main objectives, the East Indies, impossible.

It is my understanding that the American carriers WERE the main target at Pearl Harbor, but unfortunately for the Japanese, they were out to sea at the time, which left only the battleships as targets.


Logically, if the loss of battleships was irrelevant at Pearl, it would have been irrelevant had they been sunk in the middle of the Pacific. Likelier still, the battleships would not have even been targeted at Midway.


No, not logically. I wasn’t talking about the importance of the weapon - the day of the battleship was over. I was refering to the mindset and strategic thinking of the brass, With Pearl a done-deal, those whose thinking was “battleships duking it out” were taken right out of the strategic decision loop. The reason Nimitz et al had the power to command that they did was that there wasn’t the competition from the “battleship” admirals. We fought with carriers and subs because* it was all we had left.* That necessitated changes in tactics, which turned out to be what we needed to do to win. If we had had battleships galore, the plans would have been influenced by their admirals, simple as that.

I don’t know if this adds anything to this debate, but it’s worth pointing out that all but two of the battleships (Arizona, Oklahoma) “sunk” at Pearl Harbor, and in fact, all but three of the ships total, were raised, repaired, modernized, and sent back into service before the end of the war. (in fact, Oklahoma was raised, but deemed too old to bother with repairing and refitting. They could have if they’d wanted to, though.)

So while the attack was in some ways a smashing success, it didn’t really succeed at all in "taking out " the US Pacific fleet. It just kind of put it on hold for a while.