Pearl Harbor: Why Was the Attack a Surprise?

On December 7, 1941, U.S. officials were anticipating hostilities with Japan—but they did not know when or where they might occur.

In November, U.S. intelligence (which had cracked Japan’s diplomatic code) revealed Japan was about to break off diplomatic negotiations.  American officials believed this could lead to Japanese military action and warned military commanders throughout the Pacific, including Hawaii. But because Japan’s military codes were still secure, they didn’t know where Japan might strike. Most thought Japan would seize oil rich British and Dutch colonies in Southeast Asia.  American forces in the Philippines could threaten such a Japanese advance—so Washington saw potential conflict there. The prospect of an assault on Hawaii, mounted across 3400 miles of ocean, seemed remote.

The timing of any Japanese offensive was also unknown. American officials hoped to delay hostilities as much as possible, while continuing to build up U.S. forces in the Pacific.

EXPLORE THE DOCUMENTS BELOW (FROM THE COLLECTIONS AT THE ROOSEVELT LIBRARY AND THE NATIONAL ARCHIVES AT COLLEGE PARK) ON THE DIPLOMATIC BREAKDOWN AND JAPAN’S SURPRISE OFFENSIVE.

In late 1940, U.S. Army cryptanalysts cracked the Japanese diplomatic code in a breakthrough known as “Magic.” Through Magic, President Roosevelt, Secretary of State Cordell Hull, and U.S. military leaders could read what Japanese diplomats were telling each other almost as fast as they could.

In this Memorandum to the President, dated July 15, 1941, U.S. Army Chief of Staff General George C. Marshall summarizes a recent Magic intercept reflecting Japan’s imminent takeover of Indo-China (Vietnam) from the French Vichy regime. Japan’s military moves in Indo-China later that month prompted FDR to increase economic sanctions on Japan and ultimately shut off all exports of oil to that country. This accelerated the diplomatic crisis.



It is important to note that Magic could only read Japan’s diplomatic code, not its military code. Discussions of the military preparations for the Pearl Harbor attack were not transmitted via the diplomatic code.

The American oil embargo shocked Japan’s leaders. In August, Prime Minister Fumimaro Konoe requested a meeting with Roosevelt to negotiate an end to the diplomatic crisis. FDR’s advisers argued that Japan and the United States needed to agree in principle on the major issues that divided them (especially Japan’s intervention in China) before any such meeting. During the weeks that followed, Japan continued to press for a meeting, but refused to commit to preliminary agreements on these issues. 

In this Memorandum to FDR, Secretary of State Cordell Hull complains about Japan’s attitude. In his response, FDR agrees that Japan must negotiate on the key issues before any summit meeting. 

Konoe's failure to reach an agreement to end the U.S. embargo led to his resignation on October 16. His successor was General Hideki Tojo, the hardline war minister. Japan’s leaders now resolved to continue negotiations until late November. If a settlement could not be reached by that time, they would likely opt for war.

As American-Japanese relations neared collapse, Tokyo dispatched a special envoy to the United States, Saburo Kurusu, to assist its Ambassador in Washington, Kichisaburo Nomura, in negotiations with the United States government. Two days before Kurusu’s first meeting with FDR, Secretary of State Cordell Hull prepared this Memorandum for the President recommending certain issues to be addressed. The meeting took place as scheduled, with little positive result. Subsequent meetings between Hull and the Japanese diplomats proved equally fruitless.

In late November, Japan proposed a six-month cooling off period. It would return conditions to where they were before the American oil embargo. The U.S. would end its embargo and Japan would agree to no further military expansion. This proposal was unacceptable to the U.S. because it didn’t address Japan’s continued military presence in China.



FDR considered making an alternate proposal for a temporary standstill for peace talks between Japan and China. During this period Japan would withdraw its forces from southern Indo-China in exchange for a partial lifting of the oil embargo. Japanese withdrawal from China was not a precondition for this agreement. This so-called “modus vivendi” proposal would buy time for the U.S. to continue increasing its military presence in the Philippines. In this telegram, FDR outlines the proposal to Winston Churchill. In a handwritten addition, he calls it “a fair proposition for the Japanese,” but adds “I am not very hopeful and we must all be prepared for trouble, possibly soon.”

FDR’s proposal for a three-month “modus vivendi” agreement with Japan was quickly challenged by members of his Cabinet and Chinese leader Chiang Kai-shek, who opposed any lifting of the oil embargo on Japan. In this telegram, British Prime Minister Winston Churchill responds to Roosevelt’s outline of the proposed agreement. Churchill expresses lukewarm support and conveys his concerns about China’s reaction. In the end, the “modus vivendi” proposal was never presented to Japan. Instead, on November 26, the U.S. presented a hard-line proposal.

In late November, U.S. officials grew increasingly convinced that negotiations would soon collapse and Japan would engage in military aggression somewhere. Through intelligence intercepts, they knew the Japanese had set a negotiating deadline of November 29 after which “things are automatically going to happen.” 



On November 26, Secretary of State Cordell Hull presented Japan’s negotiators with a blunt proposal restating America’s long-term position that Japan should withdraw its military forces from China and Indo-China, renounce the Tri-Partite Pact with Germany and Italy, promise to not attack Southeast Asia, and recognize Chiang Kai-shek as China’s legitimate leader. In return, America would end its oil embargo and normalize U.S.-Japanese trade. Japan’s negotiators viewed the “Hull Memorandum” as “tantamount to meaning the end.” On November 27, the U.S. issued a war warning to its forces in the Pacific.

In the days immediately preceding the Pearl Harbor attack, President Roosevelt grew acutely concerned about apparent Japanese preparations for some type of military offensive. Through Magic and other sources, the U.S. had learned of massive troop buildups in Indo-China. In this memorandum, dated December 1, 1941, Roosevelt instructs his top diplomats to immediately learn the intentions behind the Japanese Government’s latest move, and he discusses the obvious parallels between Japan’s actions in the Pacific and Germany’s actions in Europe. The revisions are in President Roosevelt’s handwriting.

Tensions between the U.S. and Japan grew as the Japanese massed troops in Indo-China. American officials feared this could lead to new Japanese offensives against China or other targets in Southeast Asia. On December 5, in response to FDR’s inquiry into these troop movements, Japan’s negotiators met with Secretary of State Cordell Hull and presented a note from their government claiming that the troop buildup was defensive in nature. This memorandum of their conversation reveals the deep distrust that marked US-Japan relations by this point.

On December 6, FDR made a last ditch effort to head off hostilities. He sent a message to Japan’s emperor calling on Japan to withdraw its forces from Indo-China and dispel the war clouds forming in the Pacific. Roosevelt understood his message could, at best, only delay a confrontation. That night, during a White House dinner with several dozen guests he was heard to remark: “This son of man has just sent his final message to the Son of God.” Japanese aircraft carriers were already closing in on Hawaii when FDR’s message was transmitted. Japan’s military delayed delivery of the message to the Emperor. He received it just a few minutes before the start of the Pearl Harbor attack.

President Roosevelt studied this map on December 6, 1941. The pencil notations indicate the location of a Japanese fleet that was being tracked by British and American officials. It appeared to be headed towards Thailand or British Malaya.

What FDR and these officials did not know was that another Japanese fleet—operating under radio silence—was steaming, undetected, towards Hawaii at the same time.

On December 6, Japanese officials radioed a coded message to their diplomats in Washington. It contained the first 13 parts of Japan’s 14-part response to America’s November 26 peace proposal (the “Hull Memorandum”). Japan’s entire message would be delivered to American officials on December 7. 

U.S. intelligence intercepted and decoded the message and a courier delivered it to FDR at 9:30 p.m. The decoded message also went to the Secretary of the Navy.



Though the message’s critical final part was missing, the existing text made it clear there was no possibility of a diplomatic settlement. After reading it, FDR told Harry Hopkins: “This means war.” He most likely thought the message signaled that hostilities would come more quickly than expected—likely precipitated by a Japanese strike on British or Dutch possessions in Southeast Asia. Hopkins commented that the U.S. might want to launch a preemptive strike. “No, we can’t do that,“ FDR responded, ”We are a democracy and a peaceful people. But we have a good record.”

The 14th and final part of Japan’s reply to America’s November 26 peace proposal arrived during the early morning hours of December 7. It was decrypted and delivered to FDR at 10 a.m. After reading its ominous last sentence FDR commented that he assumed this meant Japan was breaking off diplomatic relations.

This short message instructed Japan’s diplomats to deliver their country’s reply to America’s November 26 peace proposal at precisely 1 p.m. (EST) on December 7. The timing would enable Japan to present its message (breaking off diplomatic negotiations) before the start of the Pearl Harbor attack. 

This message was intercepted by U.S. intelligence and decrypted, translated, and available for distribution at 10:20 a.m. A courier delivered it to Admiral Harold Stark (Chief of Naval Operations), the White House, and the State Department. Army Chief of Staff George C. Marshall was out riding and did not read it until 11:25 a.m. Marshall recognized the potential danger in the specific delivery time and he and Admiral Stark decided to warn all American outposts in the Pacific. But their warning did not reach Pearl Harbor in time.  



At about 11:00 a.m., Japan’s embassy requested a 1:00 p.m. meeting with Secretary of State Cordell Hull to present their government’s reply (though the Japanese diplomats knew nothing of their country’s military plans). But, due to delays with translation and typing of the response, they were unable to deliver it until after 2:00 p.m. (after the start of the Pearl Harbor attack). 

At 1:47 p.m., Navy Secretary Frank Knox called President Roosevelt with the news that an attack on Pearl Harbor was underway.