Growth of U.S. Intelligence Staff, WW2

From Ultra in the Pacific: How Breaking Japanese Codes and Ciphers Affected Naval Operations Against Japan, 1941-45 (The Secret War), by John Winton (Sapere Books, 2022), Kindle pp. 182-183:

Radio intelligence had also improved (although ‘Joe’ Rochefort had left Pearl Harbor in October, 1942, the victim of Washington intrigues). The Allies had begun to realize the full potential of communications intelligence. ‘In my opinion, the value of Radio Intelligence has been demonstrated to the extent that we can never again afford to neglect it as we did before the war,’ said Commander (later Rear Admiral) Joseph N. Wenger, a member of OP-20-G, in a lecture on ‘Future Co-operation between Army and Navy’ on 1 June, 1943. ‘Furthermore, the difficulties of obtaining Intelligence have increased so greatly that we shall have to maintain an organization constantly at work on high-speed electronic equipment if we are to be prepared for any future wars. The equipment necessary to obtain Radio Intelligence is growing so complicated that we cannot wait until war comes to provide it. Certainly we cannot afford to risk another Pearl Harbor.’

By 1943 the Allies were also coming to realize the scale of resources needed for communications intelligence. For instance, the number of personnel involved, both US Navy and Army — 300 in 1939 — had risen to 37,000 by the end of the war in 1945. There was an enormous expansion, in the United States and in the United Kingdom, in courses to train large numbers of people, many of them university students, to speak or read Japanese; classicists and students in dead languages usually learned to read Japanese, while modern language students learned to speak it.

Techniques had improved in every respect of intelligence, from the interrogation of prisoners-of-war to the evaluation of aerial reconnaissance photographs (colour-blind men and women were recruited because their disability enabled them to ‘see through’ camouflage).

By 1943 the Allies began to sense they were really winning the radio intelligence war against the Japanese. As more codes were decrypted, over longer periods, the cryptanalysts believed they were at last beginning to feel their way into the Japanese mind. As the Japanese suffered defeats on land and retreated, there were more opportunities to capture documents, such as diaries, operational orders and, as from [beached submarine] I-1, actual code books.