The Trịnh–Nguyễn Civil War was a 17th-century lengthy civil war waged between the two ruling families in Vietnam, the Trịnh lords of Đàng Ngoài and the Nguyễn lords of Đàng Trong, centered in today's Central Vietnam. The wars resulted in a long stalemate and century of peace before conflicts resumed in 1774 resulting in the emergence of the Tây Sơn forces following Trịnh Lords at the time, Nguyễn Lords collapsed in Saigon in 1777. During the division of Vietnam, Gianh River was used as the de facto border between two regimes Đàng Ngoài and Đàng Trong in peacetime.

Nguyễn Phúc Nguyên was an early Nguyễn lord who ruled the southern Vietnam from the city of Phú Xuân from 1613 to 1635. During his rule, the Nguyễn established a city at modern-day Saigon. Later, his refusal to pay tribute to the court in Hanoi sparked the Trịnh–Nguyễn War.