There is a tendency to view the conflict between these doctrines as only part of the long-running tension between the Imperial Japanese Navy and Imperial Japanese Army over funding and strategy, although this is an over simplification. Members of both branches of the military supported both sides. After the military setbacks at Nomonhan and the Mongolian front in the late 1930s, many Army supporters of the "Strike North Doctrine" changed sides.
In its initial stages, the “Southern Expansion Doctrine” focused primarily on Southeast Asia. The vast majority of Japanese emigrants to Southeast Asia in the early Meiji period were prostitutes (Karayuki-san), who worked in brothels in Malaya, Singapore, Philippines, Dutch East Indies and French Indochina. However, by the turn of the century, private Japanese companies became active in trade in the region, and the Foreign Ministry had established consulates in Manila (1888), Singapore (1889), and Batavia (1909). Communities of emigrant Japanese merchants arose in many areas, selling sundry goods to local customers, and large scale Japanese investment occurred especially in rubber, copra and hemp plantations in Malaya and in Mindanao in the southern Philippines.
With increasing Japanese industrialization came the realization that Japan was dependent (and thus vulnerable) on the supply of many raw materials from overseas locations outside its direct control. The need to promote trade, develop and protect sea routes, and to officially encourage emigration to ease overpopulation came simultaneously with the strengthening of the Imperial Japanese Navy, which gave Japan the military strength to project power to protect these overseas interests should diplomacy fail.
However, World War I had a profound impact on the “Southern Expansion Doctrine”. Japan was able to occupy the vast areas in the Pacific formerly controlled by the German Empire: i.e. the Caroline Islands, Mariana Islands, Marshall Islands and Palau. In 1919, these island groups officially became a League of Nations mandate of Japan and came under the administration of the Imperial Japanese Navy. The focus of the “Southern Expansion Doctrine” expanded to include these island groups (Nan'yo), the economic and military development of which came to be viewed as essential to Japan's security.
In the 1920s and 1930s, the "Southern Expansion Doctrine" gradually came to be formalized, largely through the efforts of the Imperial Japanese Navy's "South Strike Group", a strategic think tank based out of the Taihoku Imperial University in Taiwan. Many professors at the university were either active or ex-Navy officers, with direct experience in the territories in question. The University published numerous reports promoting the advantages of investment and settlement in the territories under Navy control.
The Anti-London Treaty Faction (han-yohaku ha) of the Treaty Faction within the Japanese Navy set up a “Study Committee for Policies towards the South Seas” (Tai Nan-yo Hosaku Kenkyu-kai) to explore military and economic expansion strategies, and cooperated with the Ministry of Colonial Affairs (Takumo-sho) to emphasis the military role of Taiwan and Micronesia as advanced bases for further southern expansion.
The Japanese government sponsored several companies, including the Nan'yo Takushoku Kabushiki Kaisha (South Seas Colonization Company), the Nanyo Kohatsu Kabushiki Kaisha (South Seas Development Company), the Nan'yo Kyokai (South Seas Society), and others with a mixture of private and government funds for development of phosphate mining, sugar cane and coconut industries in islands and to sponsor emigrants. (Japanese Societies) were established in Rabaul, New Caledonia, Fiji and New Hebrides in 1932 and in Tonga in 1935.
The success of the Navy in the economic development of Taiwan and the South Pacific Mandate through alliances among military officers, bureaucrats, capitalists, and right-wing and left-wing intellectuals contrasted sharply with Army failures in the Chinese mainland.
In order to evade monitoring by the western powers, they were camouflaged as places to dry fishing nets or coconut, rice or sugar cane farms and Nan'yo Kohatsu Kaisha (South Seas Development Company) in cooperation with the Navy assumed responsibility for construction.
This construction increased after the even more restrictive London Naval Treaty of 1930, and the growing importance of military aviation led Japan to view Micronesia to be of strategic importance as a chain of “unsinkable aircraft carriers”, protecting Japan, and as a base of operations for operations in south-west Pacific.
The Navy also began examining the strategic importance of Papua and New Guinea to Australia, aware that Australian annexation of those territories was motivated in large part in the attempted to secure an important defense line.
The Doctrine also formed part of the doctrinal basis of the Greater East Asian Co-Prosperity Sphere proclaimed by Prime Minister Konoe Fumimaro from July 1940. Resource-rich areas of Southeast Asia were earmarked to provide raw materials for Japan's industry, and the Pacific Ocean was to become a “Japanese lake”. In September 1940, Japan occupied northern French Indochina, and in November, the Pacific Islands Bureau (Nan'yo Kyoku) was established by the Foreign Ministry. While the events of the Pacific War from December 1941 overshadowed further development of the "Southern Expansion Doctrine", the Greater East Asia Ministry was created in November 1942, and a Greater East Asia Conference was held in Tokyo in 1943. During the war, the bulk of Japan's diplomatic efforts remained directed at Southeast Asia. The "Southern Expansion Doctrine" was brought to an end by Japan's defeat in World War II.