From Code Girls, by Liza Mundy (Hachette, 2017), Kindle pp. 200-201:
The Navy women had just missed taking part in the code-breaking triumph at Midway, but ten months later they were fully embedded for, and actively engaged in, the other great code-breaking event of the Pacific naval war. On April 13, 1943, a message came through along the E-14 channel of JN-25, addressed to “Solomons Defense Force, Air Group 204, AirFlot 26, Commander Ballale Garrison Force.” The code breakers weren’t able to recover the whole message right away, but the fragments they did recover suggested that the commander in chief of the combined fleet—Admiral Yamamoto himself—was headed to Ballale Island (now Balalae) on April 18. Intelligence officers concluded that this was an inspection tour.
The initial break was made in the Pacific, but Washington also got busy, recovering additives and code groups so that blanks could be filled in. More messages were intercepted, and the fast-working, far-flung teams exchanged findings. Among those digging out code recoveries was Fran Steen from Goucher. The inter-island cipher JN-20 “carried further details” about Yamamoto’s upcoming trip, so Raven’s crew of women were busy as well, adding facts and insights. Together the code breakers were able to reconstruct Yamamoto’s precise itinerary, which called for a day of hops between Japanese bases in the Solomon Islands and New Britain. Their translation concluded that the commander would “depart RR (Rabaul) at 0600 in a medium attack plane escorted by six fighters; arrive RXZ (Ballale) at 0800”; depart at 1100 and land at RXP (Buin) at 1110; leave there at 1400 and return to Rabaul at 1540, traveling by plane and, at one point, minesweeper. He would be conducting an inspection tour and visiting the sick and wounded.
It was an extraordinary moment. The Americans knew exactly where the enemy’s most valuable—and irreplaceable—naval commander would be, and when. Yamamoto was known for punctuality. Far above the pay grade of those working additive recovery, Nimitz and other top war officials decided Yamamoto would be shot down. It was not a light decision, assassinating an enemy commander, but they made it. The itinerary, as one memo later put it, signed the admiral’s “death warrant.”
In what was known as Operation Vengeance, sixteen U.S. Army fighter planes, Lockheed P-38s, went into the air on April 18, taking off from a Guadalcanal airfield. They knew Yamamoto would be flying in a Japanese bomber the Americans called a Betty, escorted by Zero fighter planes. The Americans calculated their own flight plan to meet the route they anticipated Yamamoto would be taking, planning to encounter him over Bougainville. They flew for so long that the pilots were getting drowsy; the white coastline of Bougainville was racing beneath them when one of the pilots broke radio silence and shouted, “Bogeys! Eleven o’clock!” There they were, on the horizon: six Zeros, two Bettys. The Japanese did not see the Americans at first, but once they did, the escorting Zeros moved to block the U.S. fighter planes, firing so the bombers could escape. There was a hectic battle in which it never became clear who had shot down whom, but one Betty bomber plummeted into the trees, the other into the surf. Yamamoto’s body was found in the Bougainville jungle, his white-gloved hand clutching his sword.
Cheering broke out at the Naval Annex when they heard the news. The architect of the Pearl Harbor attack was dead. The payback felt complete.