In the 1930s Japanese military planners considered two potential threats to the Home Islands. The first one, for many years regarded as the most serious (although largely overestimated), was the Russian long-range bombers that could reach Tokyo from their bases in the Vladivostok area in just a few hours.
At some point the Japanese estimated there were as many as 380 such aircraft deployed in the Soviet Far East. It was believed that the most effective way of neutralizing that threat was a preemptive land operation targeting the Soviet bomber bases. The offensive was to be launched in the early stages of a potential conflict in order to deny the Soviets long-range air strike capability. The task was assigned to the Kwantung Army (Kanto-gun) in Manchuria.
The second threat was the U.S. Pacific Fleet, although the Japanese believed it had only a very limited capability to launch an invasion of the Home Islands. Nonetheless, plans were put in place to repel the potential attack by the US Navy using the forces of the Imperial Japanese Navy (Dai-Nippon Teikoku Kaigun), as well as the Imperial Japanese Army (Dai-Nippon Teikoku Rikugun). In the event of a U.S. attack Navy and Army bombers and torpedo aircraft were to strike the invasion fleet (especially aircraft carriers and transports) at a stand-off distance from Japan’s home waters. The airborne strikes were to be followed by naval operations against the enemy invasion force in which the US Navy warships would be either destroyed or forced to retreat. Finally, the Imperial Japanese Army units were tasked with neutralizing any attempted amphibious landings launched by enemy vessels that had managed to penetrate the first two defensive layers. At the same time Japanese long-range bombers would attack air bases in China to deny the Americans their use to support the invasion force.
The early months of fighting in the Far East seemed to prove that attack was indeed the best way of defense. Lightning-fast march of the Japanese forces through South East Asia and the Pacific pushed back the enemy forces far away from the Home Islands and with it the threat of a surprise attack against Japan. The Japanese still considered the possibility of the US Navy carriers launching strikes against targets on the Japanese soil, but they believed those would be sporadic and rather limited in scope. In any event, the land-based fighter aircraft and anti-aircraft artillery units were deemed an effective and adequate protection against such attacks.
Discussion of the country’s air defense system was largely overshadowed by preparations for an all-out war against the Western powers. On November 4, 1941, during a meeting with the Army and Navy top brass, Japan’s Prime Minister and Army Minister (rikugun daijin) Hideki Tojo said:
In making air defense preparations, the first consideration must be given to invasion operations to be launched by the Army and Navy, especially their air forces. In other words, preparations for Homeland air defense must not interfere with the operations of our armed forces overseas. The strength currently available for Homeland defense is composed of approximately 300 aircraft, of which some 100 belong to the Army and the other 200 to the Navy, and approximately 700 antiaircraft weapons, 500 Army and 200 Navy, committed to the immediate defense of strategic points. Though small in number, these aircraft and antiaircraft weapons have been recently readied for action and training in their use is now being conducted.
I do not think the enemy could raid Japan proper from the air immediately after the outbreak of hostilities. Some time would elapse before the enemy could attempt such raids. I believe that enemy air attacks against Japan proper in the early stages of the war would be infrequent and would be carried out by carrier-based planes. If it should become possible for the enemy to raid Japan from bases in the Soviet Union we might face considerable danger, but I think that this is not likely in the early stages of the war.