The bad blood is mutual. Mao's armies stood aside while the NRA tried to hold off the Japanese, with the sole intention of weakening the NRA. They could have prevented several blood baths.
Chiang Kai-Shek's policy was to prioritize the fight against the Communists over the fight against the Japanese. He only agreed to a united front with the Communists against Japan after he was kidnapped by some of his own generals, who forced him to talk with the Communists.[0]
No. As the sibling comment points out, Chiang ignored the Japanese invasion until he was forced by his generals to take action. After that the much smaller Communist forces (diminished after multiple purges and military campaigns by Chiang's Nationalists) fought a guerilla war against Japan, which is the only thing they could do with the forces at their disposal. Meanwhile Chiang had a proper army, and as such fought proper battles against the Japanese.
Chiang Kai-Shek's policy was to prioritize the fight against the Communists over the fight against the Japanese. He only agreed to a united front with the Communists against Japan after he was kidnapped by some of his own generals, who forced him to talk with the Communists.[0]
0. https://en.wikipedia.org/wiki/Xi%27an_Incident