By one of the war’s most brilliant feats of intelligence work, Cmdr. Joseph Rochefort at Pearl Harbor used fragmentary Ultra decrypts to identify Midway as Nagumo’s objective. On 28 May the Japanese switched their naval codes, which thereafter defied Rochefort’s cryptographers for weeks. By miraculous luck, however, this happened just too late to frustrate the breakthrough that betrayed Yamamoto’s Midway plan.
