You're asking a lot of questions here, and some of them are loaded - a little context is helpful here.

The Second Sino-Japanese War took place in Mainland China and Burma, with full scale war dating 1937-1945. The First war dates back to 1894-95, where a politically divided Qing Dynasty was defeated by the rapidly modernizing Japanese. Japan would continue to make further political and commercial demands, aided by the post-WWI German Empire.

China is a big place, and Japan was hungry for new resources. Chinese forces were spread too thin by infighting: Think of America, but divided into a bunch of Texases and Alaskas, all fighting amongst themselves for dominance. In addition, Japanese military doctrine regarded the Chinese natives as subhuman - they effectively slaughtered their way through the country's kitchens and storehouses. This was compounded by American General Joseph Stilwell, who was notoriously distrustful of WWII allies in the China-Burma-India Theater : Stilwell convinced President Roosevelt to send an ultimatum to Chiang Kai-shek, threatening to "end all American aid unless Stilwell was placed in unrestricted command of all forces at once". Naturally, against the leader of all China, Stilwell's posturing did not go well - Major General Wedemeyer replaced him immediately.

Analyzing the casualties, lower estimates rate the Chinese military casualties at 1.3 million killed, 1.7m wounded. Japanese estimates state 0.5 million deaths and 1.9m wounded. It's worthwhile to remember that around 17-22 million Chinese civilians were found dead in the aftermath.

When it came head-to-head, neither Military had significantly stronger might. The Japanese forces were permitted to kill indiscriminately, while the Chinese forces faced the twofold task of defending their citizens and themselves.