Did China have a very large and well equipped military during and before WW2?

Question:

who were the Japanese really fighting before the Americans in WW2? as they were mostly invading China during WW2 before the Americans came to the outer islands...how and where they fought any military?

and why the Japanese didnt help out with fighting the Russians? (As they could have come from the other side?

Answer:

China was to Japan what Russia was to Germany. China was too big, with too harsh of terrain, too long of a front, and too many people for Japan to subdue. The Chinese government (Nationalist) under Chiang Kai Shek joined forces with Communists under Mao Zedong to fight the Japanese, mostly using Guerrilla warfare (which would later haunt the Nationalists).

The Japanese already had their hands full with China, French Indochina (where the Vietnamese were already running an insurgency), Australians and Kiwis on New Guinea, and the Americans in the South Pacific that they did not need to complicate things further by declaring war on Russia. Russia declared war later to gain some spoils from the Japanese--in part to right what they lost in the Russo-Japanese war years before.