In the late 1910s, China was greatly divided by different military leaders without a proper central government. Sun saw the danger of this, and returned to China in 1917 to advocate unification. He started a self-proclaimed military government in Guangzhou (Canton), Guangdong Province, southern China, in 1921, and was elected as president and general.

In 1923, he delivered a speech in which he proclaimed his Three Principles of the People as the foundation of the country and the Five-Yuan Constitution as the guideline for the political system and bureaucracy. Part of the speech was made into the National Anthem of the Republic of China.

To develop the military power needed for the Northern Expedition against the militarists at Beijing, he established the Whampoa Military Academy near Guangzhou, with Chiang Kai-shek as its commandant and with such party leaders as Wang Ching-wei and Hu Han-min as political instructors. The Academy kept running during the rest of the Republic of China and continues to serve as a major military school in the People's Republic of China today.