Wasn't it was an enemy-of-my-enemy alliance that occurred in the context of the Japanese invasion, and both sides were planning on picking up their civil war where they left off, after the Japanese had been removed?
No, the First United Front was formed in 1924, the purge of Communists by the Nationalists started with the Shanghai massacre 1927 and the Long March began in 1934, before the beginning of the Second Sino-Japanese War in 1937.