I've watched some new documentaries on Chinese forces in WWII and got some new appreciation for the Nationalist troops. The general impression that Chinese forces were not doing a lot in WWII is just plainly wrong. It's true that when the Japanese started the invasion on 09-18-1937, the Chinese forces in Manchuria didn't do much except pulling back to the south of the Shanhai Pass. However, once the Japanese passed the same line, the Nationalists were fighting hard. Yes, they lost most of the fighting at the beginning (1937-1938), but no one can say they didn't try. And also yes, they lost battles because of conflicts within their own troops, but no one can say they gave up easily.
The impression that Nationalists were not fighting came from decades of communist education on the Mainland, in an effort to glorify what the communists were doing and to villainize the Nationalists. I myself got the same education when I went to grade school there. I think now they are trying to make thing right again since a lot of new documentaries show how much the Nationalists were contributing to the fighting.
Especially, in the middle and later stages of the war (after 1939) when the Japanese advanced to Central China, the Nationalists were very effective at fighting the Japanese. For instance, the Chinese fought the Japanese to a stalemate in Shangsha in 3 major battles (the battles of Changsha) in the span of 4 years (1939-1942). Note that this was NOT some guerilla fighting involving a company or two, but huge fighting involving army groups that totaled in close to half million troops from both sides in each one of these battles (details in websites below)
Further, the Chinese fought Japan on a second front in Burma and India. Nationalists sent the Chinese Expeditionary Forces to Burma/ India in 1942 and fought the Japanese in Southeastern China in 2 campaigns for 3 years 'til the end of the war. This was also large battles between army groups. China and Allied forces (mainly British colonial forces) lost badly in the 1st campaign. However, Chinese forces alone came back and defeated the Japanese thoroughly and effectively in the second campaign. The Chinese forces were able to push Japanese almost completely out of Burma in 1944/45.
Also, the main reason that Japan decided to invade southeast Asia was to gather supply for their fighting in China. Years of fighting in China had stretched Japan's fighting capability to the limit, they could not sustain the fighting and had to find some place to get more resources. That's why they invaded southeast Asia. While they were doing that, they realized that the US became a big threat to their goal of total control of the Southeast Asia. that's why they decided to attack Pearl Harbor. By opening multiple fronts, Japan effectively signed its own death warrant. And all this was because of fighting in China. If China didn't resist like many thought, Japan could have finished the fighting in China much quicker and would have had no need to conquer southeast Asia so early. they could easily gather resources in China and use it to conquer Southeast Asia very easily. The fact that they could not do it demonstrates that China was effectively fighting them and cause their original plan to falter.
The China theater is absolutely critical to the world in WWII because fighting against the Chinese forces had absorbed so many Japan's war fighting capabilities, including troops and supplies. This means significantly less fighting capabilities of the Japanese on the Pacific islands. If you are familiar with the Pacific theater in the WWII, you'll know that most of these islands were not defended by a lot of Japanese troops. Although these soldiers fought bravely, they almost always faced overwhelming number of Allied troops. This is because the majority of the Japanese forces were trapped in China. Imagine what would happen if China gave up fighting easily as many have thought. Japan would be able to divert majority of its forces to fight in the Pacifics. This means hundreds of thousands, even millions, of troops defending an island, not mere thousands that faced Allied invasion. In reality, some of the most ferocious fighting in the WWII occurred on Pacific islands against the Japanese. Imagine the casualties that the Allied forces would have suffered if majority of the Japanese forces could pull out of China and concentrate on those islands IF China was not fighting effectively.
As you can see above, in the middle of the WWII (1939-1942), Japan and China fought huge battles in Central China totaling millions of troops and China was able to fight Japan to a stalemate and to keep huge amount of Japanese forces and supplies trapped in the area for years on end. This caused Japan dearly and made Japan unable to focus on the Pacific islands. This was the main reason that the Allied forces could advance in the Pacific relatively easily. After the defeat in the 1st campaign in the China/Burma/India theater, China almost completely lost contact with the outside world. At this point, the U.S. understood the importance of the China theater in their effort to defeat Japan in the Pacifics. So the US sent Air Corp to fly tens of thousands of sorties from India to China to keep the supply going so that China could continue fighting. From the effort made by the Allies to aid China, we can definitely tell how important the China theater was to the Allied forces in the WWII.