The A to Z Challenge asks bloggers to post 26 posts, one for each letter of the English alphabet, in April. Most of us choose to make these posts on a particular theme. My theme for 2022 is Codebreaking in World War II, which fits with the topic of the novel that I’m writing. Visit every day (except Sunday) in April for a new post on my topic.

Y is for Admiral Isoroku Yamamoto

Admiral Yamamoto was in charge of the Japanese naval fleet during much of World War II. He was the strategic thinker behind the successful attack on Pearl Harbor. Japan couldn’t keep up with the United States in ship production, so it needed to destroy as many ships as possible in the first moments of the war.

The fact that the attack on Pearl Harbor was a surprise was an intelligence and codebreaking failure, but it’s not at all clear that it was a preventable one. It used to get overblown into a conspiracy theory that the Japanese were successful only because American officials chose to allow the attack to happen as a way of making war more palatable to the American public. That’s a racist theory that posits that the Japanese couldn’t possibly have pulled off such a brilliant attack. These days, American historians seem more likely to focus on the brilliance, especially of Admiral Yamamoto.

By December 7, 1941, the Americans had broken Purple, the Japanese diplomatic code. But diplomats weren’t sharing (and probably didn’t know) details of a surprise attack.

Alice Driscoll and her team broke the Japanese naval code in the 1930s. But then, the Japanese changed the system.

In December 1940, both code and cipher were changed, to a system the Allies called JN-25B; the team stripped the additives and built a partial bank of code words. Then, in early December 1941–days before Pearl Harbor–the additive books were changed. The codebooks were not. The U.S. Navy was able to recover a certain amount of the new system–but not enough–before the attack on Pearl Harbor happened and all hell broke loose. (p. 83, Code Girls by Liza Mundy)

If Pearl Harbor showed deficiencies in codebreaking and in how intelligence was distributed to field commanders, among other things, the death of Admiral Yamamoto demonstrated how much progress was made in American cryptanalysis once the war began.

After a reported (but exaggerated) successful attack on Milne Bay of Papua New Guinea in the spring of 1943, Admiral Yamamoto planned a visit to Japanese airbases.

To pass on this news with a morale-raising visit to his frontline pilots, Yamamoto schedule a flight to Ballale and Shortland islands. His exact itinerary was learned from a deciphered message three days before the trip. Determined to get the man who had planned the attack on Pearl Harbor, the Office of the Secretary of the Navy made a decision. On April 18 American P-38s intercepted and shot down the bomber carrying Yamamoto to his troops. (p. 73, MacArthur’s Ultra: Codebreaking and the War against Japan, 1942-1945 by Edward J. Drea)

This codebreaking feat had lots of moving parts. It started with an initial break made by codebreakers in the Pacific theater of a partial message. Codebreakers at the Naval Annex in Washington DC filled in more words using deeper recovery techniques. Other messages were identified as relevant and broken. The combined work led to the precise itinerary that made it possible to shoot down the plane carrying Admiral Yamamoto. (p. 200, Code Girls by Liza Mundy)

Had you heard this story before? It was a new one to me when I started getting interest in codebreaking in World War II.