Japan’s war with China, in particular the “Three All’s Policy” (Kill All, Loot All, Burn All) officially known in Japan as the “Burn to Ash Strategy”, and Japan’s occupation of French Indochina led to America’s scrap metal and oil embargo of Japan in July of 1941. Japan relied on America for 90% of its tin, steel and oil. Emperor Hirohito directed negotiators to find a compromise with the US by 1 November or he would order the Japanese military to make preparations to seize the oil fields, rubber plantations, and tin mines of the Dutch East Indies. Before this could be done, Malaya, a British possession had to be cleared, as did the Philippines, a US Commonwealth. In order to protect these offensives from counterattack by the US Pacific Fleet, Adm Yamamoto envisioned a surprise attack on its anchorage at Pearl Harbor.
On 1 Nov, the Japanese Embassy in Washington DC reported no progress in the negotiations, and Hirohito gave his approval for preparations for war with the Netherlands, Great Britain and America, with the caveat that if a breakthrough at the table was made, the preparation would be called off. Aware that the Americans and British were tracking their carriers via radio transmissions, the Japanese immediately implemented a long prepared “deception and denial” plan that included a massive increase in fake radio transmissions. It threw the US Navy intelligence sections across the Pacific into chaos. That day, CMDR Joseph Rochefort, lead cryptanalyst of the US Navy’s Hawaii station, reported that the Japanese changed the call signs of every ship in their fleet, and his section was trying to sort everything out. That afternoon, the US Pacific Fleet went on the first of many alerts over the next month. The next day the Japanese increased the encryption of Rochefort’s “baby”, the Japanese “Flag Officer’s Code”, in addition to the Main Fleet Cipher JN-25. At the morning briefing the unorthodox but brilliant Rochefort had the unenvious job of telling Admiral Kimmel that he had “lost” four Japanese aircraft carriers. (He had actually lost six: he had mistakenly placed two in the Marshall Islands. All six were enroute to the Kurile Islands for a “fleet exercise”, actually to conduct rehearsals and prepare for the attack on Pearl Harbor, but they wouldn’t know that for another two weeks.)
In response, Kimmel ordered another alert. Additionally, he assumed the Philippines would most likely be a target of any initial Japanese attack (He was not wrong, just not right, ask an intelligence officer to explain the difference), so he also ordered the formation of the Pacific Escort Force to convoy merchantmen and freighters from Pearl Harbor to the Far East, as his brethren were doing in the Atlantic. That evening, the last non-convoyed ship reached its destination when Wake Island was reinforced by a detachment from the 1st Marine Defense Battalion led by Major James Devereux, bringing the garrison of the tiny atoll to just under 400 men.