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. (September 2017)
Japanese economic presence and political interest in Manchuria had been growing ever since the end of the Russo-Japanese War (1904–1905). The Treaty of Portsmouth that ended the war had granted Japan the lease of the South Manchuria Railway branch (from Changchun to Lüshun) of the China Far East Railway. The Japanese government, however, claimed that this control included all the rights and privileges that China granted to Russia in the 1896 Li–Lobanov Treaty, as enlarged by the Kwantung Lease Agreement of 1898. This included absolute and exclusive administration within the South Manchuria Railway Zone. Japanese railway guards were stationed within the zone to provide security for the trains and tracks; however, these were regular Japanese soldiers, and they frequently carried out maneuvers outside the railway areas. There were many reports of raids on local Chinese villages by bored Japanese soldiers, and all complaints from the Chinese government were ignored.
Meanwhile, the newly formed Chinese government was trying to recover the rights of nation. They started to claim that treaties between China and Japan were invalid. China also announced new acts, so the Japanese people (including Koreans and Taiwanese at this time) who settled frontier lands, opened stores or built their own houses in China were expelled without any compensation. Manchurian warlord Zhang Zuolin tried to deprive Japanese concessions too, but he was assassinated by the Japanese Kwantung Army. Zhang Xueliang, Zhang Zuolin's son and successor, joined the Nanjing Government led by Chiang Kai-shek from anti-Japanese sentiment. Official Japanese objections to the oppression against Japanese nationals within China were rejected by the Chinese authorities..
The 1929 Sino-Soviet War (July–November) over the Chinese Eastern Railroad (CER) further increased the tensions in the Northeast that would lead to the Mukden Incident. The Soviet Red Army victory over Zhang Xueiliang’s forces not only reasserted Soviet control over the CER in Manchuria but revealed Chinese military weaknesses that Japanese Kwantung Army officers were quick to note.
The Soviet Red Army performance also stunned Japanese officials. Manchuria was central to Japan’s East Asia policy. Both the 1921 and 1927 Imperial Eastern Region Conferences reconfirmed Japan’s commitment to be the dominant power in Manchuria. The 1929 Red Army victory shook that policy to the core and reopened the Manchurian problem. By 1930, the Kwantung Army realized they faced a Red Army that was only growing stronger. The time to act was drawing near and Japanese plans to conquer the Northeast were accelerated.
In Nanjing in April 1931, a national leadership conference of China was held between Chiang Kai-shek and Zhang Xueliang. They agreed to assert China's sovereignty in Manchuria strongly. On the other hand, some officers of the Kwantung Army began to plot to invade Manchuria secretly. There were other officers who wanted to support plotters in Tokyo.