鄭阮紛爭
Trịnh Sâm mobilized the Royal (Trịnh) 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 (modern-day Huế) 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.