Short answers, pulled from the story.
Nanshin-ron was Japan's Southern Expansion Doctrine, which argued that Southeast Asia and the Pacific Islands were Japan's sphere of interest and that their potential value for economic and territorial expansion was greater than anywhere else. The doctrine first appeared in Japanese political discourse around the mid-1880s and evolved over six decades into the strategic foundation for Japan's World War II campaigns in Southeast Asia and the Pacific.
Nanshin-ron became official policy after 1935 and was formally adopted as national policy at the Five Ministers Conference of 1936, attended by Japan's Prime Minister, Foreign Minister, Finance Minister, Army Minister, and Navy Minister. The adoption was formalized under the Toa shin Chitsujo, the New Order in East Asia, with a stated resolution to advance southward peacefully.
Nanshin-ron, the Southern Expansion Doctrine, argued that Japan should expand into Southeast Asia and the Pacific Islands, while Hokushin-ron, the Northern Expansion Doctrine, argued for expansion into Manchuria and Siberia. Hokushin-ron lost favor following military setbacks at the Battle of Khalkhin Gol in Mongolia, the quagmire of the Second Sino-Japanese War, and Japan's urgent need for the oil and resources that only southern expansion could provide.
World War I gave Japan control of former German Pacific territories, including the Caroline Islands, the Mariana Islands, the Marshall Islands, and Palau. In 1919, these became a League of Nations mandate under Japanese administration, expanding the geographic scope of the Southern Expansion Doctrine and placing a vast stretch of the Pacific under Imperial Japanese Navy authority.
Nanshin-ron provided the strategic foundation for Japan's wartime campaigns in Southeast Asia and the Pacific, beginning with the Malayan campaign of December 1941 and the Dutch East Indies campaign of January 1942. Japan targeted oil, rubber, and tin from Southeast Asia to supply its industry and aimed for the Pacific Ocean to become a Japanese lake, a strategy halted by Allied forces at the Battle of Midway in June 1942.
The Japanese government sponsored three key bodies: the Nan'yo Takushoku Kabushiki Kaisha (South Seas Colonization Company), the Nan'yo Kohatsu Kabushiki Kaisha (South Seas Development Company), and the Nan'yo Kyokai (South Seas Society). These organizations developed phosphate mining, sugarcane, and coconut industries in the Pacific mandate territories using a mixture of private and government funds, and established Japanese societies in Rabaul, New Caledonia, Fiji, New Hebrides, and Tonga.