China was both an indirect cause of the Japanese attack on Pearl Harbor and the Southern area advance, and a major ally in the Asia Theater.

In the case of the former, jingoistic rhetoric from both China and Japan intensified relations between the two countries, resulting in conflict after the Marco Polo Bridge incident followed by the Shanghai Incident. The Japanese expected to capture Nanjing, the Chinese capital at the time, within two weeks. The Chinese, however, thanks to German and later Soviet/American aid, resisted Japan's advance quite stiffly-in fact, this defense is widely considered to be have been a tactical error, as Chiang Kai-Shek, Generalissimo of China, sent his best German-trained and equipped troops into the urban bloodbath of Shanghai, in an attempt to get Western powers-who had concessions in Shanghai-to intervene. Had he listened to his German advisers and placed his best troops in the defense line that was being built, it is likely that the Japanese advance would have been even more delayed.

The end result of this was that Japan was rapidly being drained dry of its strategic war reserves, including oil and especially scrap metal. The China lobby in the United States, combined with US fears of Japanese hegemony, applied enough pressure on the US government for the Allied powers to enact a broad-spectrum embargo on Japan until they ended their war in China. The Japanese government was (rightfully) paranoid that any attempts to end the war without a Japanese victory would result in an army coup or at the very least major civil disorder. They had to continue to getting resources to end the war in Japan's favor, and so they ended up attacking the Dutch East Indies as a result.

In the war itself, Chinese troops tied up over 2 million Japanese troops in China, including two-thirds of their heavy formations, most of their ground attack aircraft, and had also contributed troops to the defense of Burma and later its recapture. By the time of the end of the war, Chinese troops had inflicted a major defeat on Japanese troops in South China and were initializing a major operation to recapture Southern China up to Shanghai, pinning down some 1 million troops.