Article: Shih-Shan Henry Tsai writes that the Yongle Emperor sent his eunuch Yang Sanbao into Tibet in 1413 to gain the allegiance of various Tibetan princes, while the Yongle Emperor paid a small fortune in return gifts for tributes in order to maintain the loyalty of neighboring vassal states such as Nepal and Tibet. However, Van Praag states that Tibetan rulers upheld their own separate relations with the kingdoms of Nepal and Kashmir, and at times "engaged in armed confrontation with them."

Question: Why did the emperor pay a small fortune in gifts?
to maintain the loyalty of neighboring vassal states