My impression is that the Japanese saw the Chinese Theater as a conventional war. COIN was a supporting effort. I think their approach to COIN was to rely heavily on Chinese proxies. This included puppet regimes and armies in Manchuria and China proper. This approach had some merit because the Nationalist Government under Chiang never had firm control over the entire country. China was still a fragmented society and political entity.

Despite the legend of mass resistance to the Japanese, I do not think the Chinese insurgency--Nationalist or Communists-was ever seriously aimed at ousting the Japanese. Stilwell's frustration with Chiang in the regard was well known, but I don't see the Communists as being significanly more aggressive.

The Communists did not make a serious attempt to push back the Japanese after the 100 Regiments Campaign of 1940. Mao and Chiang were both focused on their decisive fight which they knew would come after the Japanese were defeated.

People also forget that Chiang came close to crushing the Communists in his Encirclement Campaigns from 1931-1934. The first efforts failed but the fifth one compelled the Communists to begin the Long March. A strong argument can be made that the Japanese saved Mao by compelling Chiang to shift military resources away from the fight in the South.