China was actually very active in the Second World War. They don't often get a lot of credit due to the fact that they were absent on the European front, but they absolutely played an integral role in the Pacific.

For one, when China was invaded in 1937, it's country was in complete shambles. The invasion happened at the time of a civil war between the Nationalists and the Communists, and China was anything but united. The Communists and the Nationalists attempted to begin an alliance to fight against Japan, but had so many ideological differences between one another that a united coalition force never worked out. As a result, most of the fighting against the Japanese was done by the Nationalists (most not all). In addition, the Nationalists did not have complete control over their portion of China, but was rather a loose string of alliances between several different warlords, many of whom had their own ideas and agendas. Japan had long desired China's rich resources, and found the perfect moment to invade a weak and divided China.

Estimates of Chinese killed during the conflict with Japan (1937-1945) range from a staggering 10-20 million. The divided Chinese fought resiliently, but were outmatched by superior technology, weapons, and leadership. However, the Chinese did succeed in getting a handful of important victories against Japan despite massive casualties. U.S. pilots that were often forced to crash land in China were rescued by Chinese citizens who were more than willing to assist them. It is because of this that many American pilots were able to walk away alive, and in many cases, continue to fight in the war. (See Doolittle Raiders)

Because of the conflict in China, Japan was unable to commit all of it's forces to fighting the United States and the British in the Pacific. They were forced to keep a decent number of their forces in China due to their refusal to surrender. In my opinion, the efforts in China played a integral role in dividing the Japanese forces, helping the United States and Britain to make the advancements in the Pacific that they did. Had the Chinese surrendered in 1937/38, Japan's forces could have easily turned towards the USSR, British India, or even the United States.

I'm no expert, but my understanding was that China was pretty disorganized and very ill equipped. China wasn't a nation so much as a fractured land ruled by local warlords following the fall of the Qing dynasty and the civil war between Communists and nationalists. Hopefully that at least partially answers your question until someone more knowledgeable can answer.