When the red army eventually reached Xi'an, it was already a spent force if not for Zhang Xue Liang's aid in medication and ammunition supplies. Generalissimo Chiang Kai Shek was subsequently kidnapped by his deputy Zhang under the influence of Yang Hucheng and Zhou En Lai. This Xi'an incident was only months away from 卢沟桥事件(marco polo incident) Historical events do not just happen. With the communist plotting behind the scene right from 九一八事件 (1931) getting the students and workers to go on street protest to stir up anti-Japanese sentiment and a boycott for japanese goods, the role of the communist leading to the war must therefore not be discounted.
the CCP gained an incredible rise in status after Xi'an, from an outlawed guerrilla party about to be annihilated to a major opposition party.
you are right about the Communists' role leading up to the war. during the 1930s, the Chinese Communists under Comintern orders were persistently working to fuel anti-Japanese sentiments, sabotage Japanese business interests and frequent murders of her citizens in China. their purpose were to provoke conflicts between China and Japan. they eventually succeeded with the lugouqiao incident.