<P> 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 Expanse) is ruled by the Trịnh Lords and Nguyễn lords in the South, called Đàng Trong (Inner Expanse) 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 . </P> <P> After the Tay Son Wars (1770--1802) and the founding of the Nguyễn dynasty, the country started getting the present shape with the center of power now switched to Huế in Central Vietnam . During French colonialism, the French divided the country into three parts, directly ruling over Cochinchina (southern Vietnam) while establishing protectorates in Annam (central Vietnam) and Tonkin (northern Vietnam). Consequently, Cochinchina was more directly influenced by French culture than the other two regions . Hanoi, being the capital of French Indochina, was the only place in Northern Vietnam with significant French influence . </P> <P> From 1954 to 1975, Vietnam was again divided into two separate nations, divided by the Bến Hải River in Quảng Trị Province at the 17th parallel . The North, ruled by a communist government, was allied with communist China and the Soviet Union, while the South had a free - market economy, quasi-democratic government and had contact with the United States, the West and Western - aligned nations . Although the nation has been united since 1975, linguistic, cultural, and other differences serve to delineate the two regions from one another, with accompanying stereotypes . </P> <P> The largest city in the North is Hanoi, the nation's capital; and the country's economical capital and largest city in the South is Ho Chi Minh City (formerly called Saigon). </P>

When did north and south vietnam become one country