The primary reason Japan was able to be defeated when they were, and possibly at all, and the primary driving force of their defeat, was absolutely China.

"75-80% of Japan's military was trapped in China for most of the war. Nationalist Chinese resistance to these Japanese advances was ineffective, primarily because the Nationalist leadership was still more interested in holding their forces in reserve for a future struggle with the Communists than in repelling the Japanese. By contrast, the Communists, from their base in north-central China, began an increasingly effective guerrilla war against the Japanese troops in Manchuria and North China. The Japanese needed large numbers of troops to maintain their hold on the immense Chinese territories and populations they controlled. Of the 51 infantry divisions making up the Japanese Army in 1941, 38 of them, comprising about 750,000 men, were stationed in China (including Manchuria). Including the strong Japanese Kwantung Army stationed in Northeast China, were pinned down. Thus Japan was able to employ only 10 or 11 divisions in the Pacific theatre, with the other five divisions stationed on Japanese islands." (Britannica)

"The scale of China's resistance destroyed Japan's strategy. At the time of Pearl Harbour 80% of Japan's troops were in China. They could never be released to form the Pacific perimeter against the US due to China's resistance. Japan launched repeated attacks in China including in 1944 using 500,000 troops in the Ichi-Go offensive. This was almost twenty five times the 21,000 Japanese troops that fought the US at Iwo-Jima or more than six times the 76,000 regular Japanese troops that defended Okinawa. Given appalling US casualties in both battles if Japan had been able to release hundreds of thousands of troops from China to defend its Pacific perimeter the total Allied victory in Asia's war at worst might not have been achieved, and at best would have involved far greater US causalities.

Unlike Hollywood China is not seeking any pre-eminent position. It states every country that participated in the greatest military conflict in human history, the World War to defeat Japanese aggression and Nazism, played a vital role. The sole reason the present generation enjoys relative peace and prosperity, and are not called upon to show the same courage as the generation of 1931-45, is because of that gigantic earlier sacrifice. But regarding such immense events there are two great truths. Individually the courage of combatants of every country participating in the great defeat of aggression and fascism was equal, and that in that struggle no country played a greater role than China." (China daily)

Japan sustained losses and casualties totalling 1.5 million in China and at the end of the war, China accepted the surrender of 1.28 million Japanese soldiers.

By comparison, the allied American, British, and Canadian forces killed, wounded and captured a total of 1.25 million of the Japanese forces. Which would mean 70% of Japanese forces were killed and captured by China. While the other allies combined only eliminated 30%. (WW2 database) (China daily) (Wikipedia)

The Soviets were the sword that decapitated the Nazis, (and to an extent the Japanese) while the China was the shield that held back/trapped the Japanese. 35,000,000+ Chinese and Soviets died holding back/crushing the fascist hordes. We owe everything to their sacrifices.

source (link no longer works due to bourgeois persecution)