Why did Kangxi decide to end the power of the three vassal kings in South China?

The Kangxi emperor decided to end the power of the three vassal kings—Wu Sangui, Shang Kexi, and Geng Jingzhong—because their virtual immunity and control over private armies in South China posed a significant threat to the central government in Beijing.1

These warlords had been granted their titles and territories for their contributions to the early Manchu conquest, but by the time Kangxi took power, their autonomy had become a menace to imperial authority. When Shang Kexi offered to retire in 1673, Kangxi seized the opportunity to challenge the formidable military strength of the remaining kings. He reasoned that because the vassal kings would inevitably rebel against the Qing dynasty, it was strategically better to forestall them by acting immediately to deprive them of their armies. This decision triggered the Revolt of the Three Feudatories, a conflict that Kangxi ultimately won, thereby securing the stability of his empire.1