As talks broke down, Admiral H. E. Kimmel and Major General W. C. Short received a warning from Washington about the potential for a Japanese attack, prompting them to heighten vigilance on November 27.

Hawaii was considered an easy assignment, and the prospect of war seemed remote. Major Short was more concerned about the risk of sabotage, so he ordered all army aircraft to be grouped for better security, inadvertently making them more vulnerable to air attacks. He also ensured the safeguarding of munitions, raised the alert status of coastal artillery, and shut down radar stations at 0700 hours.

Meanwhile, Kimmel began a rotation of carriers in and out of the harbor and established patrols with naval ships and aircraft. Vessels were instructed to be watchful for potential submarine threats to shipping. Despite these precautions, there was a prevailing belief that sabotage or even an invasion force was more likely than an air attack.

In the meantime, U.S. government codebreakers were monitoring Japanese transmissions. While still neutral, Washington agreed with London that the Allies would focus on defeating Germany first in a general war. London received three highly classified MAGIC decoders, but Pearl Harbor did not receive any.

This “Germany first” policy diverted resources that could have strengthened the Pacific Fleet to the Atlantic. Additionally, 50 lend-lease destroyers the U.S. Navy could have used in the early days were sent abroad.

Despite the looming threat of war, life at the naval base continued as usual. Ships on maneuvers typically returned in time for the weekend at their berths. Theoretically, a third of the fleet was out at any given time, though arrivals and departures occasionally overlapped.

Pearl Harbor was the most fortified U.S. naval base in the Pacific and a primary stopover on journeys from the mainland to the Orient. The base boasted a robust complement of coastal artillery, and while it housed some older aircraft, several new B-17s often flew in from the United States. Japan perceived Pearl Harbor as a significant security threat.

As the negotiation deadline neared, Japanese naval vessels discreetly left their anchorages in small groups to rendezvous at Tankan Bay in Etorofu, part of the Kurile Islands. If negotiations failed, they planned to set sail on November 26, taking a northern route to avoid accidental detection. They would refuel on December 3 and then head toward Pearl Harbor with a destroyer screen, with strict orders to sink any vessels and preserve the secrecy of this operation at any cost. Dummy transmissions would be transmitted from near the Japanese mainland to mislead Allied intelligence into believing the fleet was still in Japanese waters.

Source: Antill, Peter. “Pearl Harbor: The Day of Infamy, 7 December 1941.” Pearl Harbor: The Day of Infamy, 7 December 1941, 28 Oct. 2001, www.historyofwar.org/articles/battles_pearl_harbor.html.