While there is no doubt China contributed a lot of blood and treasure in the War with Japan, how useful were those contributions given that Japan was strangled by a the navy, bombed by the air force and finally hit with nuclear weapons all of which were delivered from the Pacific? It has been said that China tied down a lot of Japanese troops, but would those troops have made much difference given that you can only pack so many defenders onto an island and you have to have ships to move the men around? If China had fought twice as effectively or half as effectively how much difference would it have made to the outcome of the war and to the military expenditures of America and other western allies in the Pacific?
China was the "anvil" to the Americans' "hammer."
Japan had about one million troops tied up in China proper (and another million in Manchukuo), and one million in India and Southeast Asia not facing the Americans (except in New Guinea and the Philippines). This compares to the one and half million or so that the Americans fought in the Pacific (counting New Guinea and Philippines) and the two million that they feared facing on the Home Islands. Basically, the Chinese inflicted almost as many casualties on the Japanese as the Americans, with far inferior weaponry.
By early 1944, American B-29s were bombing Japan long range from Chinese airfields. The Japanese undertook the Ichigo offensive to capture these airfields, right at the time when the Americans were launching their central Pacific offensives. Japanese attempts to reinforce and resupply their forces in China by sea made excellent targets for American submarines.The Japanese were forced to fight a two front war against two different enemies, and lost.
There are troops and then there are troops. Even among the Germans, much less the Japanese, there was a relatively small set of highly-trained, high-quality, and mechanized divisions, and a much larger set of non-mechanized troops that were good for lesser duties like occupation.
A couple of interesting opinion, although fact-supported, articles, are at the blog linked to below.
The author makes the point that... "The Japanese demonstrated an ability to shatter the Chinese at will – when they could spare the effort or inclination. But the great Japanese advances against the Western Allies in Asia and the Pacific were made by the ten most well trained and equipped divisions, while the Army staff acknowledged that the ninety odd divisions assigned to China and Manchuria were both fully occupied, and incapable of contributing much of additional value. Those ten divisions were of greater importance than the other 90, and no advance was possible without them..."