Top Photo: View looking up "Battleship Row" on December 7, 1941, after the Japanese attack on Pearl Harbor. The battleship USS Arizona (BB-39) is in the center, burning furiously. To the left of it are USS Tennessee (BB-43) and the sunken USS West Virginia (BB-48). National Archives

More than 2,400 American servicemembers and civilians were killed, and dozens of planes and ships destroyed or damaged in the December 7, 1941, Japanese attack on the American naval base at Pearl Harbor. The attack, which decisively drew the United States into World War II, remains one of the worst intelligence failures in US history.

That failure was the result of several factors, including gaps in American officials’ knowledge about Japanese intentions and their inability to accurately assess signs of an impending attack based on information available to them at the time. Japan’s attack on Pearl Harbor was, in the words of historian Roberta Wohlstetter, “an attack presaged by a mass and variety of signals, which nonetheless achieved complete and overwhelming surprise.”1

In the summer of 1940, American military analysts cracked a type of Japanese diplomatic code code-named PURPLE, giving American officials valuable insights into Japanese foreign policy. By late 1941, American military analysts had been intercepting and decrypting Japanese diplomatic messages for years as part of a program codenamed MAGIC—a success so closely guarded that the American public did not find out about it until after the war. But decrypting and translating those messages was a time-consuming process, and breaking Japan’s diplomatic code was not the same thing as breaking all relevant Japanese codes.

Unfortunately for American codebreakers, Japan’s army and navy used codes that differed not only from those used by the Foreign Ministry but were different from each other as well. In both Washington and Hawaii, American analysts worked feverishly to break Japanese military codes, but by December 1941 there were still significant gaps in their knowledge. Further complicating the Americans’ efforts to read Japanese military communications was the fact that the Imperial Japanese Navy changed its codes several times in the months leading up to the attack on Pearl Harbor.

Despite their struggles to break Japanese codes, American analysts were closely tracking Japanese naval activity. In late November 1941, after Japan’s leaders made the decision to attack the United States, the task force sailing for Pearl Harbor did so under great secrecy. Operating under strict radio silence, the Japanese fleet successfully concealed its position from the Americans.

Aircraft are prepared for launch on the Imperial Japanese Navy aircraft carrier Akagi to participate in the second wave of attacks on Pearl Harbor, Hawaii. December 7, 1941. Wikimedia Commons

Writing in 2010, political scientist Robert Jervis observed that “Intelligence is a game between hiders and finders, and the former usually have the easier job.”2 This was especially true in the runup to the attack at Pearl Harbor. At this critical moment, with the attack only days away, US Navy intelligence analysts at Pearl Harbor under Commander Joseph Rochefort reported to Admiral Husband E. Kimmel, commander of the US Pacific Fleet, that they did not know the exact whereabouts of four Japanese aircraft carriers.3  

American military officials on the ground also missed the signs of an impending Japanese attack on the morning of December 7, 1941. But besides their failures to collect vital intelligence on Japanese intentions and to accurately interpret information they did have, the Americans made an even greater blunder in failing to imagine that a devastating surprise attack on a heavily defended American naval base was a realistic possibility.

In early 1941, for instance, the American ambassador in Japan, Joseph Grew, sent a brief but disturbing message to his superiors in the State Department. Grew reported that the Peruvian minister to Japan had told American diplomats that “the Japanese military forces planned, in the event of trouble with the United States, to attempt a surprise mass attack on Pearl Harbor using all of their military facilities.”4  But without adequate intelligence assets in Japan to investigate the report, and without an independent intelligence agency to collect and interpret intelligence reports from civilian and military sources, the warning went unheeded.5  

A navy photographer snapped this photograph of the Japanese attack on Pearl Harbor in Hawaii on December 7, 1941, just as the USS Shaw exploded. The stern of the USS Nevada can be seen in the foreground. National Archives (80-G-16871) 

In the summer of 1946, after months of hearings and testimony, Congress released a massive report on the attack on Pearl Harbor, ending with 22 conclusions and recommendations that aimed to prevent such an attack from happening again. While many of them dealt with the very practical needs to coordinate intelligence activities and share information, the 10th conclusion had larger implications. “Had greater imagination and a keener awareness of the significance of intelligence existed,” the report’s authors wrote, “it is proper to suggest that someone should have concluded that Pearl Harbor was a likely point of Japanese attack.”6

After the attack on Pearl Harbor, the Americans redoubled their efforts to break Japanese naval codes. Led by Joseph Rochefort, who felt personally responsible for failing to detect the Japanese attack, Navy codebreakers in Hawaii worked night and day to identify Japan’s next moves.7 After months of work, they were able to break a key Japanese naval code known as JN-25. Armed with the ability to now read some naval communications, Rochefort and his team concluded that the Japanese were planning a major attack on the American base at Midway Island. This intelligence breakthrough was instrumental in the American victory at the Battle of Midway, in which US planes sank four Japanese aircraft carriers and halted Japan’s Pacific offensive. 
 

• 1

  Roberta Wohlstetter, Pearl Harbor: Warning and Decision (Stanford, CA: Stanford University Press, 1962), pp. 1. 

• 2

  Robert Jervis, Why Intelligence Fails: Lessons from the Iranian Revolution and the Iraq War (Ithaca, NY: Cornell University Press, 2010), pp. 2. 

• 3

  Gordon W. Prange, with Donald M. Goldstein and Katherine V. Dillon, At Dawn We Slept: The Untold Story of Pearl Harbor (New York: Penguin Books, 1981), pp. 440. 

• 4

  Ambassador Grew to the Secretary of State, Tokyo, January 27, 1941, Foreign Relations of the United States (FRUS), Diplomatic Papers, 1941, Volume IV: The Far East (Washington, D.C.: U.S. Government Printing Office, 1965). (URL: https://history.state.gov/historicaldocuments/frus1941v04/d11). Accessed 9/12/2025. 

• 5

  Prange, et al, At Dawn We Slept, pp. 31-32. 

• 6

  Investigation of the Pearl Harbor Attack: Report of the Joint Committee on the Investigation of the Pearl Harbor Attack (Washington, D.C: U.S. Government Printing Office, 1946), pp. 259

• 7

  Elliot Carlson, Joe Rochefort’s War: The Odyssey of the Codebreaker Who Outwitted Yamamoto at Midway (Annapolis, MD: Naval Institute Press, 2011), pp. 221.