Did the US Have Advanced Knowledge of the Attack on Pearl Harbor?

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Podcast Transcript

On December 7, 1941, the United States and the rest of the world were shocked by a surprise attack by the Japanese Empire on the American Navy stationed in Pearl Harbor, Hawaii. 

However, in its aftermath, there have been people who have wondered and speculated that the American government knew about the attack and did nothing to prevent it as an excuse to get the United States into the war. 

Learn more about whether the United States government had advanced knowledge of the Pearl Harbor attacks on this episode of Everything Everywhere Daily.

I don’t normally delve into what would be considered conspiracy theories on this show. Many of them are absolutely ridiculous and aren’t worth the time it would take to debunk them. 

The problem is that there have been conspiracies throughout history. The assassination of Julius Caesar was a conspiracy, as was the assassination of Abraham Lincoln.  Anything that is a secret confined to a small group of people can be defined as a conspiracy. 

So, to be true to history, you have to acknowledge that sometimes conspiracies have been true. 

On the other hand, I am a big believer in Hanlon’s Razor, which, if you can remember back to my episodes on Eponymous Laws, states, “Never attribute to malice that which is adequately explained by stupidity.”

So, the big question was if the attack on Pearl Harbor was an intelligence failure or if it was allowed to happen intentionally. 

To understand why some people think it was allowed to happen intentionally, we have to go back to the American attitude toward the war before Pearl Harbor. 

Prior to the attack, public sentiment was strongly in favor of remaining neutral. 

Over 100,000 Americans were killed in the First World War, and after the war, many Americans questioned what it was for. Americans had died in a European war where European countries were fighting over European soil. 

Throughout the 1930s, isolationism and neutrality were official US policy. When it appeared that war might break out in Europe again, the US Congress passed the Neutrality Act of 1935, which banned the sale of “arms, ammunition, and implements of war” to any country that was in a conflict. 

With the outbreak of the Spanish Civil War, the Neutrality Act was amended in 1936 and 1937 to put even more restrictions on involvement in civil wars and prevented the US from even transporting anyone who might be a belligerent in any conflict. 

When Germany invaded Poland in 1939,  President Franklin Roosevelt pushed for loosening the terms so they could support France, Britian, and China. This eventually resulted in the Lend/Lease Act in 1941, which provided support for those countries. 

However, there was clear support in Congress for continuing neutrality and staying out of the war. 

In the lead-up to Pearl Harbor, the Roosevelt Administration was working under the assumption that the United States was going to get sucked into the war at some point. 

If you remember, back to my episode on the Plan Dog Memorandum, the military had put together several different war plans, known as the Rainbow Plans, based on several different scenarios. 

All of the plans were based around war with Japan, first and foremost, with the possibility of war with Germany. 

So, I don’t think there is a lot of debate about the fact that Roosevelt wanted to join the war effort against Germany and Japan, that he couldn’t do so without the direct approval of Congress, and that the United States had been planning for the eventuality of a war with Japan. 

So, with that as the base we can work from, one of the first arguments of the theory is that Roosevelt provoked Japan. The Japanese had no choice but the lash out at the United States. 

To be sure, the United States put a great deal of economic pressure on Japan in the years leading up to Pearl Harbor. 

The United States had placed an embargo on Japan, preventing the shipment of oil, gasoline, metals, and other strategic goods. Many other Western countries followed suit, which resulted in Japan losing 90% of its imported oil and three-quarters of its foreign trade. 

Without access to oil, Japan needed to find new sources of fuel for its navy and its entire economy, which was one of the reasons it set out to invade the Dutch East Indies and Malaya, where there were oil reserves.

One could make the case that the US oil embargo, and remember the United States was the largest oil producer in the world at the time, forced Japan’s hand to seek new oil sources. Japan only had three years’ worth of reserves left before they would be without fuel. 

However, that had nothing to do with the United States per se. Japan could have easily just sidestepped the United States, attacked British and Dutch interests in Asia, and not given the United States a cause for war. 

The Japanese made the decision to attack the United States to attempt a decapitation attack to prevent the United States from interfering with their invasion of Southeast Asia. 

So, I’ll give this part of the theory partial credit. 

However, this isn’t a smoking gun. Even if you attribute US sanctions on Japan as being the reason why Japan attacked the US, and that is not unreasonable, it doesn’t mean that was the intent of the sanctions or that anyone in the Roosevelt administration wanted something like Pearl Harbor to take place. 

Another thing is that while the actual attack on December 7 was a surprise, war with Japan was not a surprise for anyone. 

The documented statements of government officials in the weeks preceding the attack also indicate that they thought that things were going to come to a head soon. 

On November 25, 1941, then-Secretary of War Henry Stimson wrote in his personal diary, “[Roosevelt] brought up the event that we were likely to be attacked [by the Japanese] … the question was how we should maneuver them into the position of firing the first shot without allowing too much danger to ourselves.”

Moroever, just two days later, on November 27, Stimson informed US Pacific commanders that “negotiations with Japan appear to be terminated to all practical purposes… Japanese future action unpredictable, but hostile action possible at any moment. If hostilities cannot … be avoided, the United States desires that Japan commit the first overt act.”

There were certainly officials within the government who wanted the Japanese to attack first.

On October 7, 1940, Lieutenant Commander Arthur H. McCollum of the Office of Naval Intelligence sent a memo to Navy Captains Walter Anderson and Dudley Knox, delineating eight ways the US could provoke Japan into a war. 

The document was classified until 1994, but in it was the damning line, “If by these means Japan could be led to commit an overt act of war, so much the better.”

This idea that they felt an attack was imminent was not just a belief held in the halls of power. 

A poll conducted by the Gallup Organization conducted just days before the attack, and oddly enough, published on December 8, showed that 52% of the American public expected there to be war with Japan. 

So, again, these are pretty general claims. Even if they thought that war was coming, what about knowledge of the actual events of December 7? 

Planning for the attack began in the spring of 1941, so it would have been impossible to have had any knowledge prior to that date. 

The Emperor didn’t give final approval of the plan until December 1. 

If the United States had knowledge of the Pearl Harbor attack, they would have to have cracked the Japanese military code before the attack. 

American signal intelligence before the war was a bit of a mess and not the organization it was to become during the war. 

The Japanese diplomatic code, known as Purple, was cracked in 1940. However, the Japanese never sent any details of the Pearl Harbor attack via their diplomatic channels. 

The only hint American codebreakers had that something was up took place on December 6, the day before the attack. A fourteen-part communique was sent via the Purple code to the Japanese Embassy in Washington. 

The document contained orders for the Japanese ambassador to break off diplomatic relations with the United States. 

The first 13 pages were decrypted before news of the attack became known and were delivered to the president and other top military officials. Some who saw the document thought it was simply Japan cutting off diplomatic ties. Roosevelt, on the other hand, saw it as meaning the start of war. 

On the morning of December 7, Army Chief of Staff General George Marshall sent a warning to all US forces in the Pacific, but by the time the message reached Pearl Harbor, it was too late.

In all the years since Pearl Harbor, there hasn’t been any documentation indicating that Japan’s military code, known as JN-25, was broken before the attack, and no one has come forward to say that it had been broken.

The final theory is that Americans were aware of the fleet heading toward Hawaii because of intercepted radio signals. 

However, there was no radio traffic because the entire fleet and all planes were under radio silence. In fact, there were no radio operators on the ships because because they were all left back in Japan to send fake transmissions.

In fact, radios were disabled on most of the ships just to prevent any radio transmissions that could be intercepted.

Once the war was over, there was a lot we learned from Japanese files during the war. One of the things that was learned was that the operational security around the Pearl Harbor attack was so great that even the Germans and Italians, Japan’s supposed allies, had no idea the attack was going to take place. 

There is a lot more research that has been done on this topic than what I can cover in an episode of this podcast. However, there is an almost universal consensus among historians that no one in the US government or intelligence service was aware of the plans for the Pearl Harbor attack.

That being said, it is easy to understand why some people think that the US did have knowledge of the attack.

There was widespread belief in the government and in the public that a war with Japan was inevitable.  There were some in the intelligence services and in the military who felt that war with Japan was desirable.

The attack on the United States may have been a result of the American embargo of Japan.

In the weeks and days leading up to the attack, there were indications that something was imminent.

However, nothing has appeared in over 80 years that would indicate that there was advance knowledge of exactly what would happen and when it would.

The argument that the US knew and, in fact, did nothing to prevent the attack was first put forward in 1944 by John T. Flynn, the co-founder of the American First Committee. The isolationist group that was against the war before Pearl Harbor. 

The Pearl Harbor attack was a disaster and an embarrassment for the United States government. In the days immediately after the attack, and for several years after, there were nine different investigations that took place into the Pearl Harbor attack and how it was allowed to happen. 

In 1995, a tenth investigation took place: a Congressional investigation.

The various investigations found incompetence, problems with code-breaking, failures in communications between various branches, and severe underestimations of Japan’s capabilities. What they didn’t find, and what no one has come forward with since the attack happened, was evidence that anyone knew it was going to happen before it did.