Wasn't it the Kuomintang who did the bulk of the fighting against the Japanese invaders, while the Communists largely hung back, hoarding their strength until the KMT was too exhausted from the long struggle to resist them after the war?
Indeed. The Communists didn't fight the Japanese at all, though most Chinese believe otherwise because of state propaganda. That's why the Communists were able to win the Civil War; the Kuomintang were exhausted from fighting the Japanese, but the Communists were largely unscathed.