The Second Sino-Japanese War began in July 1937 and became subsumed into the Second World War after the attack on Pearl Harbor. American and British historians of the Second World War have traditionally neglected this area, apparently because they viewed China's participation as being marginal. This assessment is based in large part on sources written late in the war by American observers and journalists, many of whom were hostile to the Chinese Nationalist government (under Chiang Kai-shek) and sympathetic to the Communists (under Mao Zedong). The historiography has changed significantly in the past two decades as more research has been done on the subject. It has become clear that the Nationalists actually did prosecute the war as best as they could, whereas the Communists exploited the situation to build up their strength and expand their power base.

The conflict was massive and the losses appalling. At the end of the war, the Nationalist government estimated that some 12.8 million Chinese had died, 9.1 million of them civilians. Census records (not altogether accurate) suggest an overall decline of 18 million. In the course of the war, nearly 100 million people became refugees. On the other hand, Japanese deaths in China amounted to 410,000, while another 900,000 were wounded. Both sides committed atrocities, but the Japanese were certainly more systematic and deliberate in carrying them about.

China's main contribution to the Allies was tying down a large portion of the Japanese military and its resources. At the start of the Pacific War, according to Wenzhao Tao, "36 out of 62 divisions [of the Japanese army], as well as 42 out of 58 independent composite brigades, were engaged in China, while there were only 10 divisions plus three composite brigades fighting in the Pacific." By November 1943, "there were still 39 out of the total of 70 divisions, together with 11 out of 22 brigades, in China." When Japan surrendered in August 1945, "nearly 45 per cent of the total Japanese armed forces were in China (except for north-east China)." Tao observes that both Churchill and Roosevelt feared what would happen if the Japanese were able to deploy their forces elsewhere.

To put things in perspective, as late as April 1944, the Japanese army committed "approximately 500,000 troops (twenty divisions in all, representing about 80 percent of the 620,000 troops available to the China Expeditionary Army), 100,000 horses, 15,000 motor vehicles, and 1,500 pieces of artillery" to the Ichigō Offensive in southeastern China; as Hara Takeshi notes, this was "the largest number of men ever mobilized in the history of the Japanese army." S. C. M. Paine adds that the Japanese "had stockpiled ammunition for two years and aviation fuel for eight months." While the Chinese did suffer devastating losses in the end, the Japanese efforts would obviously have been better spent elsewhere (i.e., fighting the United States).

The Chinese also fought in Burma alongside the British and Americans. Chiang Kai-shek was willing to commit his best divisions and commanders to these campaigns, and they performed well given the circumstances. General William Slim offered them high praise: "I had expected the Chinese soldier to be tough and brave, but I was, I confess, surprised at how he responded to the stimulus of proper tank and artillery support, and the aggressive spirit he had shown. I had never expected, either, to get a Chinese general of the calibre of Sun [Liren]."

China's continuing resistance also undermined Japanese propaganda about creating an "Asia for Asians" through the Greater East Asia Co-Prosperity Sphere. As Ronald Spector succinctly puts it: "The Japanese claim to be fighting a war to liberate all Asians from the Western imperialists could never be given full credence as long as Asia's most populous and largest nation was ranged on the side of the Allies."

Ultimately, however, it was the Americans who dealt the decisive blow against Japan; the Chinese were unable to achieve victory on their own. Nevertheless, it is important to recognize that the Chinese, on the whole, fought mainly for their own survival. Spector concludes that "millions of Chinese did not endure the hardships and losses of seven long years of war to ensure an Allied victory but to liberate their country from the Japanese. That this liberation was achieved was, for the Chinese, the vindication of their sacrifices and the ultimate victory of their cause."

Edit: Added a few sentences and a paragraph.

Sources and further reading: Hsi-sheng Ch'i, "The Military Dimension, 1942-1945," in China's Bitter Victory: The War with Japan, 1937-1945, ed. James C. Hsiung and Steven I. Levine (Armonk and London: M. E. Sharpe, 1992), 157-84; Wenzhao Tao, "The China Theatre and the Pacific War," in From Pearl Harbor to Hiroshima: The Second World War in Asia and the Pacific, 1941-45, ed. Saki Dockrill (New York: Macmillan, 1994), 134-49; Diana Lary, The Chinese People at War: Human Suffering and Social Transformation, 1937-1945 (Cambridge and New York: Cambridge University Press, 2010); Frank McLynn, The Burma Campaign (New Haven and London: Yale University Press, 2010); Hara Takeshi, "The Ichigō Offensive," in The Battle for China: Essays on the Military History of the Sino-Japanese War of 1937-1945, ed. Mark Peattie et al. (Stanford: Stanford University Press, 2011), 392-402; in the same volume, Ronald Spector, "The Sino-Japanese War in the Context of World History," 467-81; S. C. M. Paine, The Wars for Asia, 1911-1949 (Cambridge and New York: Cambridge University Press, 2012).

Edit: u/ScipioAsina's response below mine is getting less attention and I think it better answers what OP actually asked and is, in general, more in-depth.

The Battle for China began much earlier than the rest of WWII and came out of a long struggle for the Japanese. Many consider the China War to have begun in 1931 with the Mukden Incident in which Japanese operatives destroyed a railway station at Mukden and used it as the pretense for an invasion of Manchuria, eventually resulting in the installment of the puppet-state Manchukuo. This was the first invasion of China by the Japanese and was largely unpunished. Japan continued to press into China between the period of 1931-1937. War itself began after Japanese and Chinese forces fired on each other at the Marco-Polo Bridge Incident of 1937. This violence against Japanese forces, since the Japanese claimed the Chinese were the antagonists, was grounds for a full land invasion, leading to the first battle at Peking and later to the violence at Nanjing. Chiang Kai-Shek and the chinese government, however, continued to move the capital inland until it was located at Chongqin, which was sufficiently inland to be extremely difficult for Japanese forces to reach. As such, violence continued between the Chinese and Japanese between 1937 to 1945. The Chinese could not force the superior Japanese military out of the country, although both Chiang Kai-shek and Mao's forces won victories and continued to harass the Japanese forces. Japan had spent so long propagandizing and poured so many resources into their invasion, it was impossible for the government to admit defeat and save face. A look at the Hibiya Park Incident after the Russo-Japanese war, in which the Japanese public rioted in Hibiya Park in response to Treaty terms that did not represent what they felt to be a total victory against the Russians, illustrates the possible repercussions of anything less than a total victory. Japan then invaded the rest of South East Asia in an attempt to gain resources to continue their invasion and subjugation of China. I'll oversimplify how this led to Pearl Harbor since it isn't part of the question.

China then was extremely important in the defeat of Japan since their refusal to surrender, or Japan's refusal to admit to defeat, as the China War meant that by 1941 Japan was already drained of resources. They had already been fighting for 4 years (or 10) by the time they began to fight their Americans and as such struggled to feed their war machine, while the Americans had only just begun. China then not only was essential to defeating the Japanese, since it was the factor that caused American-Japanese violence, but also managed to avoid losing long enough that victory had become impossible for the Japanese.

There were many specific battles that the Chinese won or contributed to victory, but to name them I'd have to find my source book. Speaking of, source: The Battle for China: Essays on the Military History of the Sino-Japanese War of 1937-1945, Edward Drea and Mark Peattie.

I'm a little late and there are already some pretty good answers, but hopefully you'll find this information helpful.

I’ll try to give you a brief overview of the war in China, and I’ll try to highlight a few key battles and generals that you may want to look into more on your own.

Its worth noting before getting into the World War II era conflict between China and Japan that there had been many different conflict between the two nations in the preceding years. Japan had already seized Taiwan as well as concessions in other major cites, and were also present in the Shanghai International Settlement. Japanese business owned major parts of Chinese infrastructure, most notably the railroads in Manchuria. China’s government was weak and the country had been divided between “warlords” who had only recently been forced by Chiang Kai-shek and the Kuomintang to recognize the new central government, and even then they had very little presence in some regions. This was also true in Manchuria where the Japanese already held considerably political power. In 1931 the Japanese military, without orders from the civilian government, stages in the Mukden Incident in order to seize full control of the railroads. They bombed their own railroad, blamed it on the Chinese, and seized the province. Japan later placed former Qing Emperor Puyi on the throne to legitimize the new puppet state of Manchukuo.

This started a series of conflicts that cannot be considered a war. One of the more notable campaigns during this period that you may want to look into is the Great Wall Campaign, where Japan engaged and defeated the Kuomintang armies in Manchuria to consolidate their control. Overall though, much of the fighting of this time period were with Chinese rebels who were not affiliated with the government or the Chinese Communist Party.

During this period, Chinese General Zhang Xueliang, whose father had been murdered by the Japanese, kidnapped Chiang Kai-shek, and forced him to join a United Front with the communists to fight Japan. To this day Zhang Xueliang is a national hero in China, and in western literature is often referred to as The Young Marshall.

The war truly begins in 1937 when the Japanese military, again without the support of the civilian government, stages the Marco Polo Incident (known as Lugouqiao in Chinese), where the Japanese claimed their forces were attacked near the Marco Polo bridge, and invaded China. The first significant campaign of the war was fought around Beijing and Tianjin. Zhao Dengyu was a notable Chinese general who died defending Beijing. There is a secluded memorial to him near the Marco Polo Bridge today that still attracts a lot of visitors.

The first major Chinese victory of the war was fought shortly after Nanjing at the old canal city of Taierzhuang. It was here that Li Zongren, one of China’s most formidable generals, fought the Japanese army led by Rensuke Isogai and Itagaki Seishiro. The Chinese army was nearly trapped and was about to lose all access to the railroad network. Seishiro and Isogai wanted to capture the railroads as quickly as possible to cut the Chinese off and unite the Northern and Central fronts. Li Zongren baited them into attacking Taierzhuang, surrounded their army, and destroyed them. Chong Kai-shek recognized how important the battle was, and visited the battlefield personally. He brought with him Bai Chongxi, and left Bai to aid Li in conducting the battle. Bai Chongxi and Li Zongren were close friends and allies, and their critics often call them “Li Bai” (a pun on the Tang Dynasty poet Li Bai’s name) because they were so close. The Taierzhuang was museums estimates the Japanese lost over 7,000 soldiers. Li Zongren would then pull his army back to Wuhan, buying the Nationalists time to move the capital even further inland to Chongqing. Seishiro and Isogai were disgraced and moved to positions where they could do less harm, as it was the first major engagement the Japanese had lost in the entire war. Isogai however was later made the governor of Hong Kong.

After the Japanese victory at Wuhan, the war devolved into a stalemate for years. One of the next major Chinese victories over Japan would be fought jointly with the British in Burma in 1944. The Japanese had been trying to cut of the supply routes from Burma into western China, but the campaign was an utter disaster and major victory for Nationalist China.

When Japan surrendered in 1945, they still retained must of what they had captured in the first few years of the war. China failed to dislodge the Japanese and retake major cities throughout the country side. This would prove to be a significant failure for the Nationalists, as the people living under Japanese occupations saw the communist guerrillas as the only real resistance to Japanese rule throughout the conflict, and the anti-Japanese resistance cells that the communists built throughout the war would later be turned against the Chinese government.

Hsi-Sheng Ch’I . Nationalist China at War: Military Defeats and Political Collapse, 1937-1945. Ann Arbor: University of Michigan Press, 1982.

MacKinnon, Stephen R., Diana Lary, and Ezra F. Vogel. China At War: Regions of China, 1937-1945. Stanford, Calif.: Stanford University Press, 2007.

Wasn't the whole Xi'an incident over the fact that he KMT were fighting the Communists rather than the Japanese like they were supposed to.

Excellent thread. I can't offer any answers, but I'm loving reading the responses. I've often wondered the same thing.

Would saying the Chinese are to Japan what the Russians were to Germany be an appropriate analogy?

Chinese armies in Burma were, on paper, reporting to an American General, Gen. Stilwell. Barbara Tuchman dedicates a part of her book "Stilwell and the American experience in China" to the struggles of Gen Stilwell and the meddling of Chiang Kai-shek in remote.