Uhhhh,I don't want to give away any military secrets or nutting,but Japan was a small island,and China is one BIG-ass country.
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On the mark.
Japan occupied Manchuria, Taiwan, Korea, much of the Dutch
East Indies as well as many Pacific Islands prior to Pearl Harbor.
Most of that had to be clawed back before Japanese defeat.
The geography of China is completely different and much simpler.
China shares 14,000 miles of border w/more than a dozen nations
and can be penetrated rather easily from west, south and east.