The second phase of the Expedition began in January 1928, when Chiang resumed command. By April 1928, the nationalist forces had advanced to the Yellow River. With the assistance of allied warlords including Yan Xishan and Feng Yuxiang, nationalist forces secured a series of decisive victories against the Beiyang Army. As they approached Beijing, Zhang Zuolin, leader of the Manchuria-based Fengtian clique, was forced to flee, and was assassinated shortly thereafter by the Japanese. His son, Zhang Xueliang, took over as the leader of the Fengtian clique, and in December 1928, announced that Manchuria would accept the authority of the nationalist government in Nanjing. With the final piece of China under KMT control, the Northern Expedition concluded successfully and China was reunified, heralding the start of the Nanjing decade.