I have seen many conflicting views on this topic, some have argued that the KMT were ineffective fighters against Japanese forces and tried to reserve soldiers for a future war against the communists, while the CCP played a major role as an insurgency in bogging them down.
At the same time I have also seen the opposite argument made: that the KMT played a major role in holding back Japanese forces while the CCP played a much more minor role and was the party guilty of trying to reserve their forces for a war with the KMT.
I'm curious what the modern historical consensus is on the actual roles both the KMT and CCP played in the war
Variations of this question get asked on a regular basis.
u/hellcatfighter has written an answer here, they have numerous other answers concerning the Second Sino-Japanese war which might interest you
u/parksungjun has written an answer here.
u/scipioasina has written an answer here.
I myself have written an answer here.