The Japanese began to push from south of the Great Wall into northern China and the coastal provinces. Chinese fury against Japan was predictable, but anger was also directed against Chiang and the Nanking government, which at the time was more preoccupied with anti-Communist extermination campaigns than with resisting the Japanese invaders. The importance of "internal unity before external danger" was forcefully brought home in December 1936, when Chiang Kai-shek, in an event now known as the Xi'an Incident, was kidnapped by Zhang Xueliang and forced to ally with the Communists against the Japanese in the Second Kuomintang-CCP United Front against Japan.