"Following the death of Sun Yat-sen in 1925 Chiang Kai-shek emerged as the Kuomintang's new leader. In March 1926 he began his so-called Northern Expedition to consolidate his power and, at least nominally, unify China, aims which he had achieved to some extent by mid-1928. He had accomplished this with a relatively small "national" army that answered to his KMT government by entering into alliances with various provincial warlords that left each of them with varying levels of independence. Chiang may thus have been weak, but he was clearly working tirelessly to cement central power.
Each of the remaining semi-autonomous warlords had to be cajoled, bribed, and bullied into line, clearly a long-term effort, but this did not assuage Tokyo's alarm. A weak China was part of Japan's overall strategy in Asia. Zhang Zuolin, the warlord of Manchuria (to the Chinese the Three Eastern Provinces) had failed to stop Chiang's drive and he was assassinated by the Japanese Kwantung Army in June 1928, to be replaced by his son. In September 1931 the Kwantung Army staged an explosion that they blamed on the locals and used as a pretext for aggression. Within six months the Japanese had pushed Zhang's troops, who were under orders not to resist, out of their garrisons and finally south of the Great Wall. The Japanese then established the puppet state of Manchukuo in the region.
Escalating violence in Shanghai led to Japanese bombing on 28 January 1932 and 3,000 Japanese troops fanned out to take parts of the city. The 19th Route Army put up stout resistance. In mid-February the Japanese increased their strength to 90,000, while Chiang sent his German-trained 5th Army (87th and 88th Divisions) to Shanghai. In early March the 19th Route and 5th Armies pulled back, ending the fighting. The Japanese eventually largely withdrew, but insisted that Shanghai be demilitarized, including decommissioning of the arsenal facilities there…"
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THE BATTLE FOR THE BATAAN PENINSULA, PHILIPPINES, 1941–42
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