rice, which the Formosans could market or consume themselves. By 1939, Taiwan was the seventh-greatest sugar producer in the world. The Han and aboriginal populations were classified as second- and third-class citizens. Many prestigious government and business positions were closed to them, leaving few natives capable of taking on leadership and management roles decades later when Japan relinquished the island. After suppressing Han guerrillas in the first decade of their rule, Japanese authorities engaged in a series of bloody campaigns against the indigenous people residing in mountainous regions, culminating in the Musha Incident of 1930. Intellectuals and labourers who participated in left-wing movements within Taiwan were also arrested and massacred (e.g. Chiang Wei-shui and Masanosuke Watanabe). Around 1935, the Japanese began an island-wide assimilation project to bind the island more firmly to the Japanese Empire. Han culture was to be removed, and Chinese-language newspapers and curriculums were abolished. Taiwanese music and theater were outlawed. A national Shinto religion (國家神道) was promoted in parallel with the suppression of traditional Taiwanese beliefs through the reorganization of their temples and ancestral halls. Starting from 1940, families were also required to adopt Japanese surnames, although only 2% had done so by 1943. By 1938, 309,000 Japanese settlers were residing in Taiwan. Burdened by Japan's upcoming war effort, the island was developed into a naval and air base while its agriculture, industry, and commerce suffered. Initial air attacks and the subsequent invasion of the Philippines were launched from Taiwan. The Imperial Japanese Navy operated heavily from Taiwanese ports, and its think tank "South Strike Group" was based at the Taihoku Imperial University in Taipei. Military bases and industrial centres, such as Kaohsiung and Keelung, became targets of heavy Allied bombings, which also destroyed many of the factories, dams, and transport facilities built by the Japanese. In October 1944, the Formosa Air Battle was fought between American carriers and Japanese forces in Taiwan. During the course of Second Sino-Japanese War and World War II, over 200,000 of Taiwanese served in the Japanese military, with over 30,000 casualties. In addition, over 2,000 women, euphemistically called "comfort women", were forced into sexual slavery for Imperial Japanese troops. After Japan's surrender in WWII, most of Taiwan's approximately 300,000 Japanese residents were expelled and sent to Japan. Republic of China (1945–present) While Taiwan was still under Japanese rule, the Republic of China was founded on the mainland on 1 January