naval air units. The creation of these air units had begun at the end of World War I, when plans had been drawn up for 17 of them, however these plans were not fully implemented until 1931. They were to be located at six air stations around the Japanese home islands: Yokosuka, Sasebo, Kasumigaura, Omura, Tateyama, and Kure. These units were composed of various types of aircraft which were mostly seaplanes. In absolute numbers, land-based aircraft provided the largest growth in Japaneses naval air power in the years before the Pacific War. The Circle One naval expansion program which had been formulated in 1927 and put into effect in 1931 called for the creation of 28 new air groups. Although only 14 groups were actually established by 1934, which was a response to American naval expansion under the first Vinson plan, the Circle Two program called for eight additional air groups to be created by the end of 1937. They were to operate out of six new air stations at Ōminato, Saeki, Yokohama, Maizuru, Kanoya, and Kisarazu in the home islands and Chinhae on the southern coast of Korea. Under the pressure of the second Vinson plan, initiated by the United States, the Japanese increased the momentum in building up their land-based air forces. The deadline for completion date of the aviation of the Circle One expansion moved up to 1937 and an all-out effort was also made to complete the aircraft production of the Circle Two program by the end of the same year. By the end of 1937, the navy possessed 563 land-based aircraft, in addition to the 332 aircraft aboard its carrier fleet. The navy air service had a total of 895 aircraft and 2,711 aircrew, including pilots and navigators, in thirty-nine air groups. Although, this total 895 aircraft was considerably less than total American naval air strength for the same period, Japan's land based aviation force was substantially larger. The substantial land-based air power worked to Japan's advantage when the nation went to war in 1937 with China. Expansion (1931–1937) By 1927 Japanese naval aviation had grown sufficiently in size and complexity that it was necessary to consolidate the administrative organization of the air service. The various air operations and activities during peacetime, which were divided between the Navy Ministry and the Navy Technical Department, were now merged into a single Naval Aviation Department. In 1932,