United States Preparedness
Pearl Harbor advance-knowledge debate

U.S. civilian and military intelligence forces had, between them, good information suggesting additional Japanese aggression throughout the summer and fall before the attack. None of it specifically indicated an attack against Pearl Harbor. Public press reports during that summer and fall, including Hawaiian newspapers, contained extensive reports on the tension and developments in the Pacific. During November, all Pacific commands, including both the Navy and Army in Hawaii, were explicitly warned that war with Japan was expected in the very near future. And, on the day of the attack, General Marshall sent an imminent-war warning message to Pearl Harbor specifically. In Hawaii, there were several indications of the incoming attack, but none caused increased local readiness by defenders. Had any of these warnings produced an active alert status, the attack would have been resisted more effectively and perhaps caused much less death and damage. The attack arrived at a Pearl Harbor that was in fact unprepared: anti-aircraft weapons were not manned, ammunition was locked down, anti-submarine measures were not implemented (no submarine nets, for instance), combat air patrols were not flying, scouting aircraft not in the air at first light, etc.

U.S. signals intelligence, through the Army Signal Intelligence Service and the Office of Naval Intelligence's OP-20-G unit, intercepted Japanese diplomatic traffic and had broken many Japanese ciphers, though none carried either strategic or tactical military information. Distribution of this intelligence was capricious and confusing, and did not include material from Japanese military traffic as this was not available. At best the information was (as is common in such cases) partial, seemingly contradictory, or insufficiently distributed (as in the case of the Winds Code). Warnings were sent to all U.S. forces commands in the Pacific, including the explicit war warning message in late November 1941. Despite the growing information pointing to a new phase of Japanese aggression, there was little information specific to Pearl Harbor.

American commanders were warned that tests had shown that shallow-water torpedo air launches were possible, but no one in charge in Hawaii fully appreciated the danger posed by new developments. Expecting that Pearl Harbor had natural defenses against torpedo attack (eg, shallow water), the U.S. Navy failed to deploy torpedo nets or baffles, which they judged cumbersome to ordinary operations and so a low priority. Due to a claimed shortage of long-distance planes, long reconnaissance patrols (Navy and Army Air Corps) were not being made as often as was required for adequate coverage, or as were possible. At the time of the attack, the Army, which was responsible for defense of Pearl Harbor, was in training mode rather than on actual alert. Most of its portable anti-aircraft guns were stowed, with the ammunition locked in separate armories. To avoid upsetting property owners, officers did not keep the guns dispersed around the Pearl Harbor base (i.e., on private property).

                                     Breaking off negotiations

Part of the Japanese plan for the attack included breaking off negotiations with the United States 30 minutes before the attack began. Diplomats from the Japanese Embassy in Washington, including the Japanese Ambassador, Admiral Kichisaburo Nomura, and special representative Saburo Kurusu, had been conducting extended talks with the State Department regarding the U.S. reactions (see above) to the Japanese move into Indochina in the summer.

In the days before the attack, a long multi-part message was sent to the Embassy from the Foreign Office in Tokyo (encoded with the PURPLE cryptographic machine), with instructions to deliver it to Secretary of State Cordell Hull at 1 PM Washington time (i.e., just thirty minutes before the attack was scheduled to begin). The last part arrived not long before the attack, but because of decryption and typing delays, Embassy personnel failed to deliver the message at the specified time. The last part, breaking off negotiations ("Obviously it is the intention of the American Government to conspire with Great Britain and other countries to obstruct Japan's efforts toward the establishment of peace through the creation of a new order in East Asia... Thus, the earnest hope of the Japanese Government to adjust Japanese-American relations and to preserve and promote the peace of the Pacific through cooperation with the American Government has finally been lost"), was delivered to Secretary Hull several hours after the Pearl Harbor attack.

The United States had decrypted the last part of the final message well before the Japanese Embassy managed to, and long before a finished typed copy of the decrypt was finished. It was decryption of the last part with its instruction for the time of delivery which prompted Gen. George Marshall to send his famous warning to Hawaii that morning. It was actually delivered, by a young Japanese-American cycle messenger, to Gen. Walter Short at Pearl Harbor several hours after the attack had ended. The delay was due to an inability to locate General Marshall after decryption and translation (he was out riding), trouble with the Army's long distance communication system, a decision not to use Navy facilities to transmit it, and various troubles during its travels over commercial cable facilities. Somehow its "urgent" marking was misplaced during its travels and it was delayed by several additional hours.

Japanese records, admitted into evidence during Congressional hearings on the attack after the War, established that the Japanese had not even written a declaration of war until after they heard of the successful attack on Pearl Harbor. That two-line declaration of war was finally delivered to U.S. Ambassador Grew in Tokyo about 10 hours after the attack was over. He was allowed to transmit it to the United States where it was received late Monday afternoon.