Did the US know that Pearl Harbor would be attacked? The real history that debunks the conspiracy

The US itself was still reeling from the aftershocks of the Wall Street Crash in 1929, which had condemned millions to the bleakness of the Great Depression in the subsequent decade.

As the international situation grew increasingly fractious, Congress passed legislation to enshrine US neutrality. These laws effectively proscribed any American involvement with combatant nations and imposed certain stipulations on the trade of items such as arms, moneylending and more.

Within months of Nazi Germany’s invasion of Poland, much of Europe, save for its ally, Fascist Italy, Francisco Franco’s Spain, and friendly authoritarian regimes in Central Europe, had succumbed to Hitler’s rapid militarism. Only the United Kingdom held out against the Nazi onslaught, joined in June 1941 by the Soviet Union following Germany’s invasion. On the other side of the world, Germany’s ally Japan had made an incursion into China.

In August 1941, the US embargoed the oil exports on which Japan was so dependent. Negotiations between the two nations followed, while Tokyo drew up plans to take the oil-rich Dutch East Indies and British Malaya by force.

The Japanese anticipated that this latest move would incur a declaration of war by the United States. Therefore, they resolved to wipe out the threat of immediate American retaliation by launching a pre-emptive strike at the heart of US military power in the Pacific. Japanese strategists believed the US would sue for peace rather than commit to fighting thousands of miles away in East Asia and thus enable Tokyo to consolidate its gains there.

At 7.48am on 7 December 1941, 177 Japanese warplanes appeared in the skies over the Hawaiian island of Oahu, home to the US Pacific Fleet’s headquarters at Pearl Harbor.

Two hours later, the base was a scene of devastation, with over 20 warships sunk or damaged, nearly 200 airplanes obliterated and the loss of 2,403 US personnel.

Roosevelt would describe the bombing as a “date which will live in infamy” before a joint session of Congress the following day. A formal declaration of hostilities with Japan passed almost unanimously, and so the United States joined the Second World War.

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The conspiracy theory: Roosevelt knew the attack was coming and let it happen

According to conspiracy theorists, Roosevelt’s primary objective was to keep the British in the fight against Nazi Germany. The fall of France in June 1940 made this even more pressing.

The theory takes its cue from the fact that despite its official policy of neutrality, the US was in fact flouting its own laws by aiding Britain with supplies, weapons and even escorts for British convoys evading German warships in the Atlantic prior to 7 December 1941.

Foreknowledge of a ‘surprise’ Japanese attack on Pearl Harbor provided the president with a ‘backdoor’ route to join the Second World War and thus to lend full, transparent assistance to Britain.

• Read more | A gathering storm: why did Japan attack Pearl Harbor in 1941?

What is the source of the theory?

As international relations deteriorated during the 1930s, governments invested a great deal in espionage and codebreaking.

Pertinent to the idea that the US government was not only aware that a massive Japanese attack was forthcoming but in fact welcomed it, hinges on the breaking of the ‘Purple’ cipher.

This was a secret diplomatic code Tokyo used to communicate with its embassies around the world. Purple was cracked in September 1940 by 27-year-old mathematician, Genevieve Grotjan, and this enabled the US to construct a machine to decrypt secret Japanese communications.

Journalist and author Steve Twomey explains: “It wasn’t a case of reading them weeks later, they were reading them within 24 or 48 hours. Those codes clearly indicated that something big was going to happen.”

Another source cited as evidence of a plot for entering the conflict comes from the diary of Henry L Stimson, the US Secretary of War, in an entry dated 25 November 1941.

While recalling Roosevelt’s view that Japan was likely to spring an attack any day, Stimson wrote: “The question was how we should maneuver [sic] them into the position of firing the first shot without allowing too much danger to ourselves.”

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The reasons why the theory took hold

The Japanese ambush at Pearl Harbor came as a profound and inexplicable shock to Americans.

“There were a group of New York elites, having Sunday dinner at a house in the suburbs of New York City,” says Twomey. “The telephone rang, and the person who answered it came back and told the assembled smart people that Japan had just attacked Pearl Harbor. And one of the savviest members of that group told the others: ‘don’t worry about it, it’s a hoax’.”

The public had been repeatedly told that Pearl Harbor was a nigh impregnable fortress. “It was called the ‘Gibraltar of the Pacific’, says Twomey.” Even a column on the frontpage of The New York Times’ edition for 7 December 1941 (which went to press hours before the attack) was headed by a quote from Frank Knox, Secretary of the Navy, extoling the American fleet as “superior to any.”

For some, the reality was so incomprehensible that it defied acceptance.

It was widely assumed that such a significant and strategic military base – particularly with tensions rising in the Pacific – must have been conducting routine aerial reconnaissance and would have raised the alarm about the proximity of a foreign power’s fleet long before any attack could take place.

• Read more | Alternate history: what if Japan hadn't attacked Pearl Harbor?

It later came to light that the commander of the US Pacific Fleet at Pearl Harbor, Admiral Husband E Kimmel, had neglected to take appropriate precautions – notably air searches upon receipt of a communiqué issued from Washington on 27 November to its command posts throughout the Pacific, warning about the prospect of an imminent Japanese strike.

Another factor behind this rude awakening was the perception of the enemy. “The Japanese had been repeatedly described to the American public as an inferior military power. That their planes were second rate; that their aircraft carriers were not like America’s,” says Twomey.

This propaganda was often imbued with crude tropes, as Twomey explains, it was assumed that the Japanese people “suffered – in one astounding allegation – from limited eyesight and a bad sense of balance because they had been carried on the backs of their mothers as children [which had] upset their inner ear”.

“Suddenly here were these supposedly inferior people surprising the best navy in the world at Pearl Harbor. How could that have taken place?” The answer, says Twomey, is people ratonalised there must be a conspiracy.

The evidence that debunks the theory

The Pacific had become a powder keg by 1941 and a Japanese attack was widely anticipated by the United States. But what did come as a surprise to most was the eventual target. Pearl Harbor, located approximately 4,000 miles southeast of Tokyo, was considered too distant for any major military operation.

“Everyone in the United States government from the president down expected war in the Pacific to break out at any minute,” says Twomey. “Many knew that a large Japanese invasion force had set sail and was bound for the southwest Pacific toward Malaya, Singapore, Indochina, Indonesia, and, most importantly, toward what was an American colony at that time, the Philippines.”

For Twomey, the notion that Roosevelt was hoping the Japanese would give him a reason to declare war, let alone that Pearl Harbor would be willingly sacrificed to achieve that end, simply doesn’t bear out.

Yes, the likelihood of an attack in the weeks leading up to 7 December was being discussed at the top of the US government – as Stimson’s diary entry shows. However, it was believed that this would be directed against America’s colonies in southeast Asia.

• Read more | Pearl Harbor aftermath: the fallout from the attack

The notion that war with Japan would then free up the US to throw itself unequivocally behind assisting Britain in the Atlantic is also dubious. “War in the Pacific was absolutely not what [Roosevelt] wanted … starting a war in the Pacific [meant] the US now had to be wary on two fronts,” says Twomey.

Another component to this theory suggests that British prime minister Winston Churchill knew Pearl Harbor was going to be attacked and kept silent. With the US finally in the war, a beleaguered Britain would be saved by the might of America’s military machine.

Nevertheless, Twomey points out that the incident “hardly helped the British” because “within hours and days of the attack, the US began shifting warships from the Atlantic to the Pacific” and thus the protection to British shipping that had existed prior to 7 December 1941 became depleted.

This wasn’t the only blow for Britain: on 8 December 1941, the Japanese launched an invasion of Britain’s colonies in Malaya and Hong Kong, with Singapore surrendering in February 1942. Britain’s supply lines from Australia and New Zealand were suddenly in danger.

Although a formal declaration of war had not been made prior to 7 December 1941, Twomey believes that by this stage in the war the US had “advanced far beyond the laws of neutrality in its efforts to help Great Britain” and was de facto at war with Germany.

But the most convincing argument against a conspiracy, says Twomey, is “the deliberate sacrifice of thousands of sailors and many warships just to get the United States into the war when there were so many other ways to do it”.

“There were far simpler and more productive ways … than by allowing your principal fighting force in the Pacific to be crippled.”

Steve Twomey is a Pulitzer Prize-winning journalist. He was speaking to Rob Attar for this episode of the HistoryExtra podcast, part of our Conspiracy podcast series