 Complete the following phrase:  When the Northern Expedition arrived in Nanjing, a series of anti-foreigner riots broke out in the city, with the ensuing chaos bringing the expeditions advance to a halt as Chiang blamed the Communists for instigating the incident. In April 1927, the Four Elders determined that the actions of the CCP were counter-revolutionary and urged Chiang Kai-shek to initiate a purge of the leftists, culminating in the 