The origin of the Second Sino-Japanese War can be traced to the First Sino-Japanese War of 1894-95, in which China, then under the Qing Dynasty, was defeated by Japan and was forced to cede Taiwan to her, and to recognize the 'independence' of Korea in the Treaty of Shimonoseki. The Qing Dynasty was on the brink of collapse from internal revolts and foreign imperialism, while Japan had emerged as a great power through its effective measures of modernization. The Republic of China was founded in 1912, following the Xinhai Revolution which overthrew the Qing Dynasty. However, the nascent Republic was even weaker than its predecessor due to the predominance of Chinese warlords. Unifying the nation and repelling imperialism seemed a very remote possibility. Some warlords even aligned themselves with various foreign powers in an effort to wipe each other out. For example, warlord Zhang Zuolin (???) of Manchuria openly cooperated with the Japanese for military and economic assistance.

In 1915, Japan issued the Twenty-One Demands to extort further political and commercial privilege from China. Following World War I, Japan acquired the German sphere of influence in Shandong[14](Shantung), leading to nationwide anti-Japanese protests and mass demonstrations in China, but China under the Beiyang government remained fragmented and unable to resist foreign incursions. In order to unite China and eradicate regional warlords, the Kuomintang (KMT, or Chinese Nationalist Party) in Canton launched the Northern Expedition of 1926-28. The Kuomintang's National Revolutionary Army (NRA) swept through China until it was checked in Shandong, where Beiyang warlord Zhang Zongchang, backed by the Japanese, attempted to stop the NRA's advance. This battle culminated in the Jinan Incident of 1928 in which the National Revolutionary Army and the Imperial Japanese Army were engaged in a short conflict that resulted in Kuomintang's withdrawal from Jinan. In the same year, Zhang Zuolin was assassinated when he became less willing to cooperate with Japan. . Afterwards Zhang's son Zhang Xueliang quickly took over control of Manchuria, and despite strong Japanese lobbying efforts to continue the resistance against the KMT, he shortly declared his allegiance to the Kuomintang government under Chiang Kai-shek, which resulted in the nominal unification of China at the end of 1928.

However in 1930, a large scale civil war broke out between warlords who fought in alliance with Kuomintang during the Northern Expedition and central government under Chiang. In addition, the Chinese Communists (CCP, or Communist Party of China) revolted against the central government following a purge of its members from the KMT in 1927. Therefore the Chinese central government diverted much attention into fighting these civil wars and followed a policy of "first internal pacification before external resistance"((Chinese): ??????).

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