The Xi'an Incident (西安事变 xi1 an1 shi4 bian4) occurred in Xian on December 12, 1936.
During World War 2, Generalissimo Chiang Kai-Shek, Commander in Chief of National Revolutionary Army of the Kuomintang, refused to ally with the Communist Party of China to fight against Japanese, and instead opted to fight both at once.
Generals Zhang Xueliang and Yang Hucheng of Kuomintang (with the support of communist leader Zhou Enlai), kidnapped Chiang Kai-shek and forced him to join an united front with the communists against the Japanese.
Even though the ceasefire was in effect, the armies were never under a common command and the KMT never fully cooperated.
After the war, the two generals were placed under permanent house arrest by Chiang and were brought over to Taiwan along with retreating KMT forces.