West Wind Clear: Cryptology and the Winds Message Controversy–A Documentary History. United States Cryptologic History Series IV: World War II Volume X (review)

“There was a Winds message. It meant War–and we knew it meant War.” So claimed Captain Laurance Safford, the former head of the US Navy’s codebreaking section, OP-20-G, before the Joint Congressional Committee (JCC) investigating the Pearl Harbor attack in 1945–46. He was referring to a “Winds Execute” message in a daily Japanese news broadcast which American intelligence had tried to intercept before the attack. In late November 1941, decrypted messages from the Japanese Foreign Ministry (the Gaimusho) to its Washington embassy had disclosed that the repeated phrase Higashi No Kaze Ame (East Wind Rain), or Higashi on its own, would alert Japanese embassies that relations with the United States were in danger, and that they should destroy their codes. Related phrases or words also existed for disrupted relations with Great Britain and the USSR. Senior American officers initially believed that the message would herald an almost immediate declaration of war. They therefore instigated a massive hunt for it, which became a major distraction from more productive work.

In the first 115 pages the authors scrutinise Safford’s claim and explain its signals intelligence background. They also allot over 200 pages to reproducing 56 source documents, including cryptanalytic material, translations of Japanese messages and American intelligence memoranda. They find that the only Winds Execute messages received were broadcast on 7 December 1941, some hours after the attack on Pearl Harbor, and that the code phrase used was “West Wind Clear”, referring to endangered relations with Great Britain - not the United States. Moreover, Gaimusho instructions in early December to destroy cryptographic material had made any Winds message virtually irrelevant. Safford’s evidence to the JCC was “little more than a farrago of fabrication, speculation, poor memory, rumor gathering and plain error-filled opinion” (p. 53). A claim by Ralph Briggs, a former OP-20-G operator, that he intercepted Higashi No Kaze Ame in a merchant ship broadcast is “too full of holes to hold up to much scrutiny.”

The final chapter analyses prewar American codebreaking, finding it overstretched, with too many targets, and an undue emphasis on the Gaimusho’s complex new cipher machine, codenamed Purple, which was most unlikely to contain intelligence about the Japanese army’s or navy’s war plans. The US Army’s Signal Intelligence Service had solved Purple in September 1940, but with few staff had to ask OP-20-G for help in deciphering Gaimusho messages, including those enciphered with manual systems. In fact, a British Purple decrypt of 2 December 1941 (UK National Archives, HW 1/288), which is not mentioned, revealed that Togo, the Japanese Foreign Minister, instructed their ambassador in Berlin to advise Hitler that “it is greatly to be feared that … we shall find ourselves in a state of war with BRITAIN and AMERICA … sooner than expected,” but it contained no detail.

Unfortunately, Safford contributed to a dearth of actionable naval intelligence by his management of OP-20-G. He diverted codebreakers at Pearl Harbor to attack the Japanese navy’s Flag Officers Code, despite the traffic being too light to permit breaks. He also wasted scarce staff by assigning them to crack Kriegsmarine Enigma. They were doomed to failure since, as the British told him, it could only be broken with high-speed machinery (the “bombes”). OP-20-G’s main target should have been the principal Japanese naval code (later called JN-25), but only about 20 people (out of 210) were working on it in Washington in December 1941. Postwar decrypts show that JN-25 messages had contained vital intelligence, including references to torpedo attacks flown from carriers against anchored capital ships, but they were not read...