Trịnh Sâm ruled northern Vietnam from 1767 to 1782 AD. He ruled with the title "Tĩnh Đô Vương", and was one of the last of the powerful Trịnh lords. Trịnh Sâm defeated the ancient enemy of the northern state, the Nguyễn lords in the south. The Trịnh line was separate from the royal Lê Dynasty, and the officially recognized king was Lê Hiển Tông, who continued to occupy the royal throne in Thăng Long, but without real power.
Early reign
Trịnh Sam was given rule over northern Vietnam by his father Trịnh Doanh in 1767. Five years after he took power, the Tây Sơn rebellion started in the south. During his lifetime, the Tây Sơn rebels focused all their efforts against the Nguyễn Lords, specifically against Nguyễn Phuc Thuan who had gained the throne as a young boy. As the Tây Sơn rebellion gained strength, the Trịnh saw the Nguyễn weakening month by month.
Trịnh Sâm mobilized the Royal army, and on November 15, 1774, it crossed the river into Nguyễn territory, re-igniting the Trịnh–Nguyễn War. With the Nguyễn army divided and weak, the Trịnh army captured Phú Xuân in February 1775. The army continued south capturing more Nguyễn lands and defeating some of the forces of the Tây Sơn. In the summer of 1775, one of the leaders of the Tây Sơn, Nguyễn Nhac, made a formal alliance with Trịnh Sam against the Nguyễn. Trịnh Sam agreed and gave Nguyễn Nhac a formal title as well as "regalia". The Trịnh army then withdrew back to Tonkin, left a small army in Phú Xuân.
For the remainder of Trịnh Sâm's life, the Trịnh allowed emperor Gia Long, the last surviving member of the Nguyễn Lords, to make repeated attempts to re-take the south. The Tây Sơn brothers were too busy fighting the Nguyễn loyalists to worry about the Trịnh in the north. In 1782 as Trịnh Sâm was dying, he tried to leave control over Vietnam to his son, Trịnh Cán, but his rightful heir, Trịnh Khai organized an army and fought against his half-brother. Although he was successful, this civil war fatally divided the Trịnh at a critical time when the Tây Sơn were gaining control over the south. having a tea party at Tả Vọng House.
