Did Roosevelt know about Pearl Harbor before it happened?

During the years leading up to the attack, American-Japanese relations had worsened after Japan had expanded into Manchuria in 1931 and invaded China in 1937.

Then, on 27 September 1940, Japan signed the Tripartite Pact with Germany and Italy, which formalised its alliance with the Axis powers.

As Japan advanced through Southeast Asia in 1940 and 1941, Roosevelt’s administration responded with economic sanctions, including a total oil embargo imposed in July 1941 that froze Japanese assets and cut off essential supplies of oil, aviation fuel, and scrap metal.

Earlier restrictions had already reduced exports of other critical materials to Japan.

For Japan, dependence on American oil to keep its military running left its leaders convinced that war was certain unless they could secure new sources of raw materials.

Negotiations between the United States and Japan took place throughout 1941 but failed to resolve the crisis.

In the final months of the year, Japanese leaders prepared for war and planned simultaneous offensives across the Pacific and Southeast Asia. 

At the same time, American intelligence had developed an important capability, as military codebreakers had deciphered significant portions of Japanese diplomatic codes known as “MAGIC.”

Thanks to this program, the Roosevelt administration received regular translations of Japanese diplomatic messages.

Some of them referred to Japan’s deteriorating relations with the United States and its likely decision to go to war.

Intercepts revealed that Tokyo instructed its embassies to destroy sensitive documents, which many officials viewed as a sign that hostilities might begin soon.

In addition, American cryptanalysts intercepted Japan’s “14-part message,” which broke off negotiations, as well as the famous “1 p.m. message,” which instructed Japanese diplomats to deliver it to Washington shortly before the attack.

However, these messages did not specify the exact location or timing of any attack and did not reveal Japan’s military plans. 

In the final weeks before 7 December, Roosevelt’s administration received warnings about the potential for Japanese aggression.

On 27 November 1941, the War Department sent a message to American commanders in Hawaii, the Philippines, and Panama, advising them that negotiations had failed and that they should be prepared for attack.

The message did not mention Hawaii as a specific target, and most American officials believed that the Philippines or British Malaya would be attacked first.

As confidence grew in the assumption that Japan would strike southward, preparations in Hawaii focused mainly on preventing sabotage rather than stopping an air attack. 

Among supporters of the theory that Roosevelt knew about Pearl Harbor, a common argument is that he desired a war with Japan to overcome isolationist opposition and to aid Britain in its struggle against Germany.

By late 1941, Roosevelt had already taken significant steps to support Britain, including the Lend-Lease program and the Atlantic Charter.

He had also responded aggressively to incidents at sea, such as the Greer incident in September 1941, which brought American warships into open clash with German submarines.

At the same time, polling data from Gallup surveys indicated that a majority of Americans opposed entering the war in Europe, and Roosevelt faced political limits on his ability to intervene directly.

Some theorists claim that he viewed a Japanese attack as a way to rally the nation to war, knowing that Germany would declare war shortly afterwards under its alliance with Japan. 

Among the evidence often cited to support these claims, proponents note that Roosevelt had access to messages from Japanese embassies indicating that Tokyo would soon end diplomatic relations with the United States.

They also refer to movements of American aircraft carriers, which were at sea when the attack occurred.

Specifically, the USS Enterprise and USS Lexington had been dispatched to deliver aircraft to Midway and Wake Island, which is seen as evidence that high-level officials knew of the threat to Pearl Harbor.

At the same time, USS Saratoga was in overhaul at San Diego and was not present at Pearl Harbor.

Based upon these facts, books such as Robert Stinnett’s Day of Deceit argue that Roosevelt deliberately provoked Japan into war and withheld information from commanders in Hawaii. 

However, historians who reject this theory maintain that the evidence does not really support these claims.

The movements of American carriers were routine and cannot be seen as evidence of prior knowledge.

After the war, investigations, including the Roberts Commission of late 1941 to early 1942 and the 1946 congressional inquiry, found no proof that Roosevelt or any other senior official had prior knowledge of the attack on Pearl Harbor.

These inquiries concluded that failures in intelligence sharing, poor cooperation between military branches, and complacency about Hawaii’s vulnerability all contributed to the disaster. 

At that time, intelligence remained incomplete and difficult to interpret. MAGIC provided insight into Japan’s diplomatic intentions but not its military plans.

American codebreakers had not yet deciphered the Japanese naval code, known as JN-25B, which contained details of operational orders.

As a result, commanders lacked the information needed to anticipate the strike on Pearl Harbor.

In addition, communications between Washington and Hawaii suffered from delays and confusion, which meant that warnings sometimes arrived too late to be acted upon effectively. 

Under the political circumstances of 1941, it is unlikely that Roosevelt would have deliberately sacrificed part of the Pacific Fleet.

Even if he wanted to provoke Japan into firing the first shot, it would have been unnecessary to allow a devastating attack on American soil.

Roosevelt had already taken significant steps to prepare the nation for war, including increasing defence spending and instituting the first peacetime draft.

After the attack, public opinion shifted rapidly, as Gallup polls recorded a sharp increase in support for war as Americans rallied behind the president after any clear act of aggression. 

Over the years that followed, numerous official investigations examined the Pearl Harbor attack and the events leading up to it.

Between 1941 and 1946, nine inquiries investigated the disaster, including the Roberts Commission, the Hart Inquiry, Army and Navy boards, and the congressional joint committee.

None found evidence that Roosevelt or his administration had prior knowledge of the specific attack on Pearl Harbor.

Those investigations instead cited failures in intelligence analysis and an underestimation of Japanese capabilities.

Even decades later, conspiracy theories about Roosevelt’s foreknowledge continue to attract attention because they offered a dramatic explanation for a shocking national tragedy.

The idea that leaders sacrificed American lives to achieve political goals appeals to those who distrust government power.

Nevertheless, the historical record does not support these claims. Roosevelt knew that war with Japan was likely, but he did not know when or where Japan would strike.

The attack on Pearl Harbor surprised American leaders as much as it surprised the public.