Trịnh–Nguyễn War
During the Trịnh–Nguyễn War (1627–73), the country was partitioned between two ruling Lords with the border being the Gianh River in Quảng Bình Province. The North, called "Đàng Ngoài" (Outer Realm) was ruled by the Trịnh Lords and Nguyễn lords in the South, called "Đàng Trong" (Inner Realm) or "Quảng Nam Quốc", with Lê emperors still nominally acting as head of state. The two sides ruled their own domain independent of the other, and frequently fought each other. The imposed separation encouraged the two regions to develop their own cultures.