With the continuing advance of the KMT, Zhang was forced to abandon Beijing on June 3, 1928. On the way back to his power-base in Manchuria the next morning, his train was blown up by officers of the Japanese Kwantung Army, killing him, in what is known as the Huanggutun incident. Yan Xishan's troops soon occupied Beijing, effectively dissolving the Beiyang government; unification was declared on June 16 by the Nationalists. Beijing was renamed Peiping until the end of the Chinese Civil War in 1949. Zhang's son, Zhang Xueliang, took over the National Pacification Army and retained a government in exile led by Premier Pan Fu. However, many civil servants, including former ministers and presidents, had already switched over to the Nationalist government. The United States became the first major power to switch recognition to the Nationalist government in Nanjing on October 1. Japan was the last major power to switch because they detested the anti-Japanese attitude of the KMT. Zhang negotiated with Chiang Kai-shek to end this pretense leading to the dissolution of the Beiyang government, the NPA, and the unification of China under the Nationalist flag on 29 December 1928.