Are you aware that the United States did not have an Air Force during World War II either? There was the Army Air Corps, which became its own separate force only after the war. Plus the navy had air support.

The main reason for the two separate air forces was because both services sought to expand their influence as far as possible. The IJA operated aircraft carriers - the Yamashio Maru, Akitsu Maru and Kumano Maru classes, while the IJN had infantry units, operating nearly four divisions of SNLF landing forces. Neither would have accepted the removal of any part of their forces to form a new one. This did lead to a large amount of duplication - for the most part, the land based aircraft of both services filled highly similar roles.

The IJN were at least slightly justifiable in their refusal to lose their aviation capacity. In the UK, giving the RAF and the Air Ministry control of the Fleet Air Arm resulted in it using obsolete aircraft during the early 1940s. In Germany, Goering refused to release aircraft production for the Kriegsmarine's planned carrier, and refused them aerial reconnaissance support.

It is more apt to consider independent air forces to the the exception. We look at most nations today with a separate air force and conclude this is 'normal', but in 1914 nations and armies and navies, and the idea of a separate air force was ludicrous, and of course existing institutions have an inertia all their own.

The Royal Air Force secured independent status because it had a strong leader and advocate for this, Lord Trenchard, who though an Army officer wanted an independent air force to win war by bombing: this concept reflected traditional British war policy of exerting control 'indirectly' and by raids, as an island nation. The 1918 German bombing of Britain and the weak response by the Royal Flying Corps (Army) and Royal Naval Air Service, on top of early German success in the Spring Offensive, say a political shake up that saw the independent RAF formed. It just managed to keep this status in the lean post war years.

The Luftwaffe was openly declared in 1934, after Germany had been forbade military aircraft by the Treaty of Versailles. As in many aspects of military organisation, this 'clean sheet' gave a huge advantage in throwing off pre-conceived notions about organsation, tactics and technology. Having said that, the Luftwaffe was entirely staffed by ex-Wehrmacht officers, but in the build up they became loyal to their arm. An additional factor promoting independence in Germany was that the Luftwaffe was the most 'Nazi' of the Services: Goering was the Nazis' most 'respectable' public figure in the early years, and was a famous ace who had led Richtofen's Jasta in 1918. He was also an industrialist, and used his leadership of he air force to build his own fiefdom within the Nazi state (Himmler did the same with the SS, so by 1944 you had paratroopers and anit aircraft gunners from the Luftwaffe, Waffen SS soldiers, and Wehrmacht troops, all fighting together. Such was German organisation and command, it worked fairly well on the battlefield. A Luftwaffe general, Kesselring, was supreme commander in Italy). Ironically, the Luftwaffe was almost entirely concerned with battlefield support so its independent status was rarely important.

To Japan. The modernisation of Japan's military was aggressively pursued, but the Navy and Army were far too resentful and divided to ever consider a separate air arm. Their arrangement also reflected the US, as did their potential geographic commitments, and in the US Independent Air Force advocates had been seen off and the most assertive, Billy Mitchell, court martialled.

The Army's status came, in simple terms, from its connection to the Samurai class who cowed the Japanese population for many years. The navy, on the other hand, was a modern technological service. Admiral Tojo's decisive defeat of the Russian navy in 1904 was the coming of age moment for modern Japan. Acceptance as a naval officer was extremely hard to secure, and was based on merit, exams, and tough physical tests. Navy men generally enjoyed better status in Japan, and the leadership looked down on their army counterparts as philistines.

In this rivalry was the basis of the separate air forces.

Once the military took control of the government, which it did by stages through the 1930s, it was the military that drove the manufacturers (the earliest were set up in 1919 and enjoyed some independence, Nakajima and Mitsubishi, the other manufacturers were more closely associated with either the Army or Navy, eg, Kawasaki and Kawanishi, respectively).

Each issued requirements based on their own needs, but up until about 1936 the emphasis was really on making established. conventional types to a standard to equal foreign aircraft. As a broad generalisation, the Army was influenced by French and German types, the Navy by Britain, but each Japanese factory (company) made alliances with European nations and some hired European designers.

By 1937 Japan was producing modern aircraft of indigenous design. The Navy emphasised long range given the likely demands of war in the Pacific and Asia (Mitsubishi A6M Zero, G4M 'Betty'), Army aircraft were often similar but more conventional (Ki-43 Hyabusa) or the specifications influenced by overseas developments (eg, the Kawasaki Toryu which was a twin engined fighter influenced by the Messerschmitt Bf-110 and French Poetz 630, or the fact that only the Army developed a serious water cooled engined fighter, the Ki-61 'Tony'). The Navy of course needed various specialised carrier aircraft as well, a situation that closely reflected the US experience where Corsairs and Hellcats flew for the navy, P-40s, Mustangs and Lightnings for the Army Air Force.

While probably wasteful of industrial resources, Japan's industries were never anywhere near developed enough to compete with those of Germany, the USA and Britain. Her technology was simply not sufficiently developed, not broad enough: while the Japanese could design and build an excellent plane to 1940 standard, she fell behind rapidly as metallurgy and electronics impacted aircraft design and reliability and manufacturing precision were not good enough for the 1944 standard of 2000hp engines and multi-stage turbo chargers.

Operationally, the two services managed their separate air forces to a reasonable extent. The Army-Navy divide was a massive problem for Japanese strategy overall, but in the air war it was dealt with. The Navy took over much of the effort in China in 1939 as the Army was exhausted by the Nomonhan Incident and battles with Chinese forces. The Army deployed fighter units to the naval base at Rabaul in late 1942 as the Navy suffered massive attrition trying to defend Guadalcanal in the Solomons, and the Army took over all operations in New Guinea at this time. Otherwise, most theatres were simply allocated to one service or the other.