Mitsubishi A6M Reisen Zeke Vol. I

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A separate article of the treaty was devoted to aircraft carriers. The participating nations agreed that the maximum displacement for a warship of that class must not exceed 27,000 tons and the number of main battery guns carried by such ships was limited to ten with a maximum caliber of 203 mm. No limits were imposed on smaller caliber weapons. Each of the five signatories of the treaty was allowed to convert up to two capital ships (or line cruisers that otherwise would have been scrapped) into aircraft carriers. The only stipulation was that the displacement of the units used for conversion should not exceed 33,000 tons and the number of 203 mm guns carried by the ships was limited to eight.
The Japanese admiralty were thoroughly satisfied with the possibilities that the treaty opened up for their navy. Japan had the third largest fleet in the world (after the U.S. and Great Britain), second in the Pacific and the first in the Far East. Construction and commissioning of a fleet of aircraft carriers would provide the Japanese commanders with a long-range air reconnaissance capability in support of their line squadrons. The Washington Treaty spelled doom for the ambitious battleship construction programs, but at the same time jump-started rapid development of naval aviation.
Construction of the first Japanese aircraft carrier had begun even before the Washington Treaty was signed. Hosho was launched on 13 November, 1921 at Asano Shipbuilding Co. in Tsurumi and entered service on December 7, 1922. It was a relatively small ship, displacing 7,470 tons, 167.9 m long and 18 m wide. However, Hosho was a thoroughly modern vessel. She featured a flight deck unimpeded by any superstructures, unlike the British Furious and Vindictive carriers of the same period. Although there were plans to build Hosho’s sister ship, they were abandoned in 1922 since the provisions of the Washington Treaty allowed a much better option – conversion into aircraft carriers of two large armored ships.
In March 1923 Japanese government launched a new fleet development program taking into account the outcomes of the Washington Treaty. Imperial Japanese Navy command ordered conversion of two battlecruiser hulls (Akagi and Amagi) into aircraft carriers. Akagi was laid down on December 6, 1920, followed ten days later by Amagi. In September 1923 the Great Kanto Earthquake struck Japan, which damaged the unfinished Amagi hull beyond repair. In early 1924 battleship Kaga was selected for conversion to aircraft carrier replacing the lost Amagi.
In those early days aircraft carriers were not considered true combat ships, but rather auxiliary vessels providing little more than long-range air reconnaissance capabilities for naval combat teams. Gathered intelligence would allow the commanders to make informed decisions to either engage comparable or weaker enemy, or to avoid contact in the face of superior enemy forces. This way of thinking was fully understandable in the 1920s, since carrier-based aircraft of the period were in fact land-based machines merely adapted to service at sea. Also in those days enemy force consisting of more aircraft carriers than battleships was considered less intimidating than the other way around. Following this logic, the Japanese thought it was more prudent to keep the Akagi and Kaga, even converted to aircraft carriers, than risk losing them altogether. Following the conversion, the ships entered service in 1927 (Akagi) and 1928 (Kaga).
It wasn’t long before the IJN high command fully appreciated the value of aircraft carriers in future war in the Pacific and they wasted no time placing orders for more vessels of that type. The light aircraft carrier Ryujo joined the fleet in 1933, followed by Soryu (1937) and Hiryu (1939).

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