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Japanese home island air defences of World War II

Pre-war air defenses


The Japanese government's pre-war plans to protect the country from air attack focused on neutralizing enemy air bases. Before the war it was believed that Soviet aircraft based in the Russian Far East posed the greatest threat. The Japanese military planned to destroy the air bases within range of the home islands if Japan and the Soviet Union ever went to war. While Japan had been at war with China since 1937, the Chinese were not capable of conducting air attacks on the Home Islands. As the overall threat of air attacks from the USSR or China was considered low, the Imperial Japanese Army (IJA) placed a little emphasis on forming anti-aircraft units. As late as 1939, only four anti-aircraft regiments were stationed in the Home Islands.

When the Pacific War began in December 1941, the Japanese government believed that the best way to prevent American air raids was to capture and hold the areas in China and the Pacific from which such attacks could be launched. It was expected that the Allies would not be able to re-capture these bases. However, the Japanese anticipated that the Allies might still make small-scale attacks against the home islands using naval aircraft flying from aircraft carriers. The government chose not to develop strong defenses to meet the threat of air attack as the country's industrial resources were unable to maintain offensive air forces in China and the Pacific as well as a defensive force in the home islands.

The General Defense Command (GDC) had been formed in July 1941 to oversee the defense of the home islands, but all combat units in this area were assigned to the four regional military districts (the Northern, Eastern, Central and Western districts) which reported directly to the Ministry of War. As a result, the GDC's functions were limited to coordinating communications between the Imperial General Headquarters (IGHQ)—Japan's highest military decision-making body—and the military districts. The air defense forces assigned to each of the military districts typically comprised an air regiment equipped with interceptor aircraft, a regiment of anti-aircraft guns and an air raid warning unit. Overall, the forces located in the home islands in December 1941 comprised comprised 100 Imperial Japanese Army Air Force (IJAAF) and 200 Imperial Japanese Navy (IJN) fighter aircraft, as well as about 310 Army-manned and 200 IJN anti-aircraft guns. Most of these guns and aircraft were obsolete, and their crews were poorly trained. At the direction of the IGHQ, half of the guns and aircraft were stationed to defend the Tokyo-Yokohama area, 20 percent were in the Osaka-Kobe area, a further 20 percent were split between the Kokura-Yawata and Shimonoseki-Moji areas and 10 percent were assigned to the defense of Nagoya. The air raid system mainly comprised a network of civilian and military observers in the home islands, and Imperial Japanese Navy (IJN)-controlled picket ships stationed 500-600 miles to sea. Only a small number of radar stations were operational, and their equipment was primitive.

Doolittle Raid
The quality of the air defense units in the home islands was not improved in the months prior to the Doolittle Raid on 18 April 1942. At this time, the Tokyo-Kawasaki-Yokohama region was defended by 150 anti-aircraft guns and the 50 Nakajima Ki-27 ("Nate") fighters operated by the 244th Air Group. A further 70 anti-aircraft guns and 20 fighters defended Osaka, and Nagoya was protected by 20 guns and 10 fighter aircraft. Command and control of the air defenses was fragmented, and the IJAAF and IJN did not coordinate their activities or communicate with each other. As a result, the forces were unable to react to a sudden air attack.

The Japanese air defenses failed to stop the Doolittle raiders. While a picket vessel detected the United States Navy task force on the morning of 18 April, it wasn't attacked immediately and few fighters were placed on alert as the Japanese military believed that the American ships were still too far from the home islands to be able to launch air strikes and any attack would be made on 19 April. No aircraft were patrolling over the Tokyo-Yokohama region when an observation post 70 mi from the city first spotted the incoming American North American B-25 Mitchell bombers. About 40 IJAAF fighter and scout aircraft were scrambled, but none of the raiders were intercepted until after they had dropped their bombs. Two of the Ki-27s intercepted a pair of American aircraft, but only managed to inflict light damage on one of them. The IJN also scrambled 24 Mitsubishi A6M Zero fighters and 30 Mitsubishi G4M bombers after the American aircraft arrived in the Tokyo area, but none of these aircraft made contact with the B-25s. No aircraft were dispatched to protect Kobe, Nagoya and Yokkaichi before they were each attacked by a single B-25. The anti-aircraft batteries were also not ready at the time of the raid, and scored no hits. The failure to shoot down any of the attackers over the home islands embarrassed the GDC, and the wreckage of a B-25 which had crashed in China was placed on display in Tokyo in an attempt to cover up the poor performance of the air defenses.

Japan's air defenses were expanded following the Doolittle Raid. As immediate measures, two batteries of anti-aircraft guns and a squadron of fighters were recalled from the South West Pacific and Burma theaters and assigned to the Eastern District Army. For the longer run, the Army decided to expand the number of fighters stationed in the home islands to 400, and increase the number of anti-aircraft guns to 1,900. The organisation of the fighter forces was also improved, with three wings of IJAAF aircraft being formed between April and May; the 17th Air Wing was assigned to the Eastern District Army, the 18th Air Wing was assigned to the Central District Army and the 19th Air Wing operated under the Western District Army. Most of the units assigned to these wings were training formations which were only required to operate in the air defense role during emergencies.

Middle years of the war
The Japanese leadership took few further steps to improve the country's air defenses during 1943. The IJAAF and IJN were focused mainly on attempts to continue offensive operations against the Allies, and officers in these services generally regarded air power as being ill-suited for defensive tasks. The two services also believed that it it was not necessary to develop new fighter aircraft capable of effectively operating at high altitudes, and little attention was paid to preparing for protracted campaigns.

Following the Doolittle Raid, the next air attacks on Japan were made against the Kuril Islands in mid-1943. The liberation of Alaska's Attu Island in May 1943 during the Aleutian Islands Campaign provided the USAAF with bases within range of the Kurils. As part of the preparations for the liberation of Kiska Island in the Aleutians, the Eleventh Air Force conducted a series of raids against the Kurils to suppress the Japanese air units stationed there between July and September 1943. The USAAF attacks were broken off for five months following a raid on 11 September 1943 when nine of the 20 B-24s and B-25s dispatched were lost, but raids by US Navy PBY Catalinas continued. In response to the American attacks, the IJN established the North-East Area Fleet in August 1943, and in November that year Japanese fighter strength in the Kurils and Hokkaidō peaked at 260 aircraft. The Eleventh Air Force resumed its offensive in February 1944 after it had been reinforced with two squadrons of P-38 Lightning escort fighters, and it continued to attack targets in the Kurils until June 1945. While these raids caused little damage, they forced the Japanese to divert large numbers of soldiers to defend their northern islands against a potential United States invasion.

American attacks from China
In April 1943 Japan's intelligence services reported that the United States had begun building Boeing B-29 Superfortress bombers which were to be used to attack the home islands. At this time, it was expected that the USAAF would begin making attacks on the home islands from bases in China in the third quarter of 1943. When these attacks did not eventuate, the Japanese military determined that the USAAF would have about 50 B-29s in China by April or May 1944. In response to the threat of these attacks, the Japanese military sought to increase the production of 120mm anti-aircraft guns and heavily armed fighters capable of operating at high altitudes; while the 120 mm guns began to be installed around key areas from early 1944, the improved fighter aircraft never entered mass production.

Further steps were taken to prepare for B-29 raids in the first half of 1944. In March 1944 responsibility for the air defense of Korea, Taiwan and the Ryukyu, Bonin, and Volcano Islands was transferred from GDC to other headquarters; this change was made to allow the command to focus on defending the core areas of the Home Islands. The headquarters of the IJA and IJN also established agreements to cooperate during defensive operations around the home islands, but the Navy only assumed responsibility for defending the ports and naval facilities. The Japanese military also began transferring fighter aircraft to the home islands from China and the Pacific during the early months of the year. The three IJAAF air brigades stationed in Honshū and Kyūshū were expanded to air divisions between March and June (these were designated the 10th, 11th and 12th Air Divisions). By late June the air defense units in the home islands were assigned 260 fighters, and could draw on approximately 500 additional aircraft during emergencies. Additional anti-aircraft gun batteries and searchlight units were also established to protect major cities and military bases. The GDC's authority was strengthened when the army units in the Eastern, Central and Western military districts were placed under its command in May. The IJN defensive fighter units stationed at Kure, Sasebo and Yokosuka were also assigned to the GDC in July, but cooperation between the GDC's Army units and the much smaller number of naval units was poor. Despite these improvements, Japan's air defenses remained inadequate as few aircraft and anti-aircraft guns could effectively engage B-29s at their cruising altitude of 30000 ft and the number of radar stations capable of providing early warning of raids was insufficient.

Organisational change placeholder section
The Air Training Army was redesignated the 6th Air Army in late December 1944; at this time several air training divisions were formed. While this army was intended to participate in the defense of the Home Islands and attack the Marianas, it proved too weak for these missions. Accordingly, in February 1945 the 6th Army Army became part of the strategic reserve to be used only in the event of an Allied invasion. Ahead of the invasion of Okinawa, in March 1945 the army was transferred from the Kanto area to Kyushu and assigned to the Combined Fleet. A new IJAAF fighter wing was established in the Kanto area to replace the 6th Army Army.

A major reorganiation of Japan's air defences came into effect on 15 April 1945. On this day all IJAAF and IJN air defense units were belatedly placed under a single command when the Air General Army was formed under the command of General Masakazu Kawabe. The 1st Air Army, which had previously been a training command, also became an operational headquarters on 15 April and was assigned the 10th Air Division.