1936: The Xi'an Incident
Zhang Xueliang and Yang Hucheng were two key Chinese warlords in Shaanxi and Gansu.
Chiang Kai Shek ordered them to carry out another extermination campaign against the communists. However, tensions were building between China and Japan, who had begun invading Northern China. Zhang and Yang contacted the Communists, setting up an armistice agreement and alliance with the Red Army against Japanese invasion.
Chiang Kai Shek was enraged by their actions and on October 22nd, Chiang flew to Xi'an with the aim of convincing Zhang and Yang to hold back the Red Army.
However, Chiang was kidnapped by Zhang's troops in December 1936 for a period of 2 weeks. It is believed that as part of the ultimatum for his release, he was forced to agree to a united front with the Communists to fight a full-scale war against Japan.
The Communists were willing to do so as the Soviets had persuaded them to unite with the Kuomintang against Japan, alongside military aid provided by the USSR. This is because Japan was a consistent threat in that region, and the USSR realised that helping China to fight Japan was the right thing and would benefit them as well.