"Considering the fact that China was invaded and occupied, you'd think they would have fought harder against the Japanese, much like the Russians, Free French and Poles did against the Germans."
you can make the argument that Russians only won by luck -- if the Nazi's had not miscalculated the Russian winter, the war would have been different.
Poles? Poles lost it within 1 week.
France? 3 weeks.
Let's not forget that Japan's army when WWII started is one more the most technologically advanced army around. US military was ranked as number 35 if I recalled (it rose to #1 after the war). The population in China was literally resisting the Japanese with sticks and stones.
The Japs bolst they could take China in 3 months, 3 YEARS later they still couldn't take Shanghai. Japanese forces suffered huge losses in Shanghai after Chiang puts up a fight (which is more or less the reason why Nanjing got massacred, as Chiang retreated to Chongqing and did not defend Nanjing).
The facts is, without Russians or Chinese or the British or even the Canadians, the US wouldn't be able to win against the Nazis or the Japanese. If Japan had successfully taken China in 3 months, WWII would have been a lot different. WIth China's enormous resources, Japan would have all the natural resources it need to take the fight to mainland USA. They know in order to control the world, they'll have to control Asia, and the key to Asia is China.
The point I've been trying to make, perhaps badly, was that China "punched way below it's weight" with regards to the war of 1937-1945. The army that invaded Korea in 1950 was not only willing, but eager, to offer battle. The change in offensive spirit cannot simply be chalked up to mere 'revolutionary fervor'. The Nationalists and Communists during the Second World War avoided battle, husbanding resources for a war against each other, despite the fact that Japan threatened China with extinction.
As for the Russians 'getting lucky', that's a fallacy. The Nazis did, of course, ignore the rasputitsa (as the Russians call the seasonal floods), but in the end, what saved Russia was the fact that germany simply ran out of soldiers. Germany could not continue to advance into the Soviet Union and still maintain enough cohesion and concentration of force to hold onto thier gains. Allied aid, especially American, kept the Russiams on their feet until they were ready to take the fight to scattered nazi armies at the end of an extremely long supply line.
The Japanese found themselves in a similar position. They simply could never muster enough troops to both occupy China in any meaningful way and so the war there became little more than the parading of armed men. The Chinese, if suitably motivated, could have pushed the Japanese out of China at any given moment, I believe, Chinese numbers and Allied supply making up for Japanese "technical superiority" (remember, this was basically the same Japanese army that had lost to the Russians in 1939, who were not all that technically advanced except in armor. Russian numbers and concentration overcame Japanese skill and technology).
You say the allies would not have won without the Chinese, but I say it's more like they won DESPITE the Chinese.
At the end of the day, the Russians, French, Poles, etc, fought, and usually well, unlike the Chinese.