I think this quote from Max Hasting's All Hell Let Loose is very telling: "China was merely a great victim, second only to Russia in the scale of its sufferings and losses, while denied the consolation of any redemptive military achievement." China suffered terribly during World War II, with popular estimates of fifteen million dead as a result of Japanese action.

Throughout the war, America dedicated vast resources to bolster Chinese defence against Japan, and built airfields in China to launch USAAF operations. Unfortunately this proved ineffective, and America's desire to turn China into a major power proved impossible.

China was a divided country, with Mao Zedong's communist forces in the north opposing Chiang Kai-shek's Nationalist regime. Chiang had a large army, however a corrupt and ineffective organisation meant they had little success against Japan. They were immensely undersupplied, and ill equipped to fight. Mao on the other hand was far more focused on an eventual conflict with Chiang, and has been accused of forming a truce with Japanese and continuing to trade with them. Although Chiang devoted resources to resisting Mao, he did far more to stop Japan's conquest, with Nationalist forces recording 3.2 million casualties against the communist forces 580,000. Despite this, the war effort in China did impact Japan's resources, and cost them 202,958 dead form 1941-1945.

Of course China's civilian population were affected the most, baring the brunt of Japan's many warcrimes and crimes against humanity, from the use of biological warfare and mass starvation, to the widespread torture and rape. Examples include 200,000 dying in Yunnan from a Cholera epidemic caused by Japanese released cholera bacteria, and the conscription of over 100,000 'comfort women'. Japanese actions became more barbaric as their war efforts else where began to fail.

To recap, China was a very divided nation, and thus could not provide much resistance to Japan's occupation of more than half the country. For the most part, China's 'role' was just as a victim of Japanese aggression.

Sources: My knowledge of China only comes from general WWII books, (Beevor's Second World War and Hasting's All Hell Let Loose) as most of what I have read is on Germany in WWII.