After Yuan Shikai's death in 1916, China was politically fragmented. Its Beijing-based government was internationally recognized but virtually powerless; regional warlords controlled most of its territory. In the late 1920s, the Kuomintang, under Chiang Kai-shek, the then Principal of the Republic of China Military Academy, was able to reunify the country under its own control with a series of deft military and political manoeuvrings, known collectively as the Northern Expedition. The Kuomintang moved the nation's capital to Nanjing and implemented "political tutelage", an intermediate stage of political development outlined in Sun Yat-sen's San-min program for transforming China into a modern democratic state. The political division in China made it difficult for Chiang to battle the communist People's Liberation Army (PLA), against whom the Kuomintang had been warring since 1927 in the Chinese Civil War. This war continued successfully for the Kuomintang, especially after the PLA retreated in the Long March, until Japanese aggression and the 1936 Xi'an Incident forced Chiang to confront Imperial Japan.