Look at the map, Japan didn't even occupied 1/10 of China territory after 15 yrs invading China.
Interesting isn't it? Since the overwhelming bulk of the Chinese population at that time was in the coastal areas. There were nothing but ethnic tribes and outposts in the interior at that time.
I must say tho, Japan did pretty well considering we were at war with the Chinese, Philippines, Americans, French, British, India, Dutch, all at once.
Had the Imperial Navy, and Army coordiinated their forces better...well, that's just hypothetics now. lol.