MacArthur's ULTRA

Codebreaking and the War against Japan, 1942-1945

Edward J. Drea

Cracking the enemy's radio code is a task so urgent and so difficult that it demands the military's best minds and most sophisticated technology. But when the coded messages are in a language as complex as Japanese, decoding problems multiply dramatically.

It took the U.S. Army a full two years after the attack on Pearl Harbor to break the codes of the Japanese Imperial Army. But by 1944 the U.S. was decoding more than 20,000 messages a month filled with information about enemy movements, strategy, fortifications, troop strengths, and supply convoys.

“A must for anyone with more than a superficial interest in World War II or military history. This is the first comprehensive examination of General Douglas MacArthur’s use of ULTRA. Drea’s use of primary sources and secondary Japanese material results in a balanced picture that gives both sides of the story. Particularly fascinating is his description of the Japanese codes themselves and how they were decoded by MacArthur’s Central Bureau.”


“Drea’s excellent study presents a detailed illustration of the interplay between intelligence-gathering and operational planning and uses one of history’s most successful commanders as its model.”

Publishers Weekly
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In MacArthur's ULTRA, historian Edward Drea recounts the story behind the Army's painstaking decryption operation and its dramatic breakthrough. He demonstrates how ULTRA (intelligence from decrypted Japanese radio communications) shaped MacArthur's operations in New Guinea and the Philippines and its effect on the outcome of World War II.

From sources on both sides of the Pacific and national security agency declassified records, Drea has compiled a detailed listing of the ULTRA intelligence available to MacArthur. By correlating the existing intelligence with MacArthur's operational decisions, Drea shows how MacArthur used—and misused—intelligence information. He tells for the first time the story behind MacArthur's bold leap to Hollandia in 1944 and shows how ULTRA revealed the massive Japanese mobilization for what might have been (had it occurred) the bloodiest and most protracted engagement of the entire war the Allied invasion of Japan. Drea also clarifies the role of ULTRA in Truman’s decision to drop atomic bombs on Japan in 1945, and concludes that ULTRA shortened the war by six to ten months.

About the Author

Edward J. Drea is chief of the Research and Analysis Division at the U.S. Army Center of Military History in Washington, D.C. He is author of The 1942 Japanese General Election and recently contributed a chapter to Intelligence and Operations, edited by Michael Handel. Drea is fluent in Japanese.

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