When American fighter pilots shot down Japanese Adm. Isoroku Yamamoto over the jungles of Bougainville in April, 1943, they chattered so much about the attack over their radios that the Japanese immediately suspected the United States had broken their naval code.

Tokyo changed the code, and it took the Americans four months to crack the new one, according to once-secret U.S. documents that have just been turned over to the National Archives by the supersecret National Security Agency. Through most of April, all of May, June and July and part of August, 1943, Japan's naval message traffic went undeciphered by the Americans, who at that critical stage of the war were relying on interceptions of the Japanese naval data to plan their strategy in the Pacific.

This new information is contained in a top-secret message of March 15, 1945, in which the U.S. Army Air Corps in the Pacific is told the upcoming itinerary of an unamed Japanese vice admiral. The message suggests that the vice admiral be left alone because he's "not worth the loss of the code."

"Past experience when message intercepted of scheduled flight Japanese Admiral Yamamoto proved disastrous," reads the 1945 message just released to the Archives. "Allied pilots in radio breach while waiting for Admiral's plane gave Japanese tipoff on compromise of their codes. Japanese naval code changed immediately resulting in four-months silence Japanese naval messages. Suggest no attack unless all security precautions to insure no tipoff to Japanese regarding source this information."

Also just released to the Archives is the 1943 cable sent by the codebreakers to the Army Air Corps on Guadalcanal detailing the trip Adm. Yamamoto was about to make to the Japanese-held islands of New Georgia and Bougainville in the Solomon Islands where the fiercest fighting of the Pacific war was then taking place.

The cable lists precisely the four places and times Yamamoto would stop on his Solomon Islands visit. At the end of the cable is a succinct one-line message: "Tally Ho. Let's get the b------."

On April 18, Yamamoto's shiny new bomber and six fighter escorts were ambushed over Bougainville by at least 10 American fighter planes. The bomber crashed in flames in the jungle, killing Japan's foremost naval strategist and leading war hero.

When the Japanese were told of Yamamoto's death two months later, the entire country went into mourning. Not only was Yamamoto the architect of the attack on Pearl Harbor, he was also the strategist who sank the British battleships Repulse and Prince of Wales in one of the worst naval defeats in English history. Yamamoto was given Japan's highest decoration by Emperor Hirohito.

At the time Yamamoto's death was announced in the United States, he was described in the American press as a warmonger who hated America. He was said to have written a friend that "I am looking forward to dictating peace to the United States in the White House in Washington."

Long after the war ended in 1945, it turned out that Yamamoto was a Japanese dove who had warned the army warlords against going to war with America, which he described as a "sleeping giant." Appalled at the ignorance of the warlords, Yamamoto wrote of what would be involved in a war with America. In warning what such a war would entail, he wound up his letter this way: "We would have to march into Washington and sign the treaty in the White House."