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.

X is for X List

“X List” was a name that American intelligence officers gave to a captured Japanese document.

Issued by the Japanese Navy as “A List of Ship Names for Communication Security at Sea,” it contained the radio call signs for all vessels listed as well as the name, speed and tonnage. (p. 228, The Other Ultra by Ronald Lewin)

Not only was this helpful intelligence information, the X List also gave the Americans more confidence in their own intelligence. There were 1720 large (over 1000 gross tons) merchant and transport ships, called marus, on this list. The Americans already knew about all but 13 of them.

That knowledge came from breaking the Japanese code used by radio operators on marus in early 1943.

The staff at Arlington Hall, along with a companion unit in Australia, began concentrating on a system known as 2468, the “water-transport code” used by the Japanese Army to route its supply ships…. (p. 224, Code Girls by Liza Mundy)

With an assist from the British and another from an American officer working in Australia, a small group of codebreakers at Arlington Hall (three men and four women) worked for more than 24 hours straight to break the system.

The break into 2468 was one of the most important of the war. It was every bit as vital as the breaking of Enigma or the Midway triumph. The 2468 code routed nearly every single maru making its way around the Pacific to supply the Japanese Army. As with Japanese Navy vessels, many marus sent a daily message giving the exact location where they would be at noon. The information would be turned over to American sub commanders…. The U.S. military employed ruses so the Japanese wouldn’t know the maru sinkings were the result of a broken code. American planes would be sent up so it would look as though they had spotted the maru from the air. The Japanese sent messages saying they thought coast watchers–spies along the island coasts–were to blame, which the Arlington Hall code breakers read with glee. (p. 227, Code Girls by Liza Mundy)

USS Barb SS-220

With the intelligence derived from 2468, U.S. submarines, which had been largely ineffective in the giant expanse of the Pacific, became very effective indeed. Ultra was the name given to intelligence produced by American codebreakers from Japanese radio signals.

When surrender came, the marus were mainly a memory and–thanks in large measure to Ultra–no major merchant fleet in history had been so mercilessly savaged.

In other words, of the 1720 marus on the X-List, nearly all had been sunk by the end of the war.