Trịnh – Nguyễn War The Trịnh – Nguyễn War ( ; 1627 – 73 ) was a long war waged between the two ruling families in Vietnam . Cooperation . Both the Trịnh and the Nguyễn families were descended from close friends and aides to the hero - Emperor Lê Lợi who freed Vietnam from Chinese rule and started the Lê Dynasty in 1428 . By 1520 a succession of weak kings had brought the country into a state of civil war ( see Lê Dynasty ' s civil war ) . For the next 20 years the Trịnh and Nguyễn families fought as allies against the usurper Mạc Đăng Dung . In theory , they both were fighting on behalf of the Lê Emperor ( Lê Trang Tông ) but in reality , the Emperor was a figurehead with little or no power . Apart . The prime mover in the period from 1525 onwards was Nguyễn Kim . His daughter married the young head of the Trịnh family Trịnh Kiểm . Around 1530 , the rebels were forced into exile in Lan Xang ( modern - day Laos ) but they gathered a new army and captured some southern provinces . In 1545 , Nguyễn Kim was assassinated and his son - in - law , Trịnh Kiểm , took control over the Royal army . 13 years later ( in 1558 ) Trịnh Kiểm gave the rulership over the southernmost province of Quảng Nam to Nguyễn Hoàng , the son of Nguyễn Kim and his wife ' s brother . The ill - will between the two families dates from around this time . For the next 55 years , Nguyễn Hoàng ruled Quảng Nam . He gradually asserted his control over the province and extended his control south into the remaining Champa lands . Periodically , he sent military forces north to help the Trịnh in their long fight against the Mạc Dynasty . In 1570 Trịnh Kiểm died and was succeeded by his second son Trịnh Tùng , a vigorous leader who captured Hanoi from the Mạc king in 1572 . However , the Mạc king ( Mạc Mau Hiep ) recaptured the city the next year . 20 years later , Trịnh Tùng , again captured Hanoi and executed Mạc Mau Hiep in 1592 . In 1593 , Nguyễn Hoàng went personally to the court , bringing money and an army to help destroy the remaining Mạc armies . Once the Mạc were defeated , the Trịnh became increasingly unhappy with the independence of Nguyễn Hoang who ruled as an independent prince in the south . For reasons that are unclear , in 1600 the old Nguyen ruler broke relations with the Trịnh court and titled himself as the " Vương " ( a term meaning prince or king ) . Hoang died in 1613 . The new leader of the Nguyễn , Nguyễn Phúc Nguyên , continued his father ' s policy of defiance , but also initiated friendly relations with the Europeans sailing into the area . A foreign trading post was set up in Hội An . By 1615 the Nguyễn were producing their own bronze cannons with the aid of Portuguese engineers . The first campaign . In 1620 , Nguyễn Phúc Nguyên officially refused to send taxes to the court in Hanoi . A formal demand was made to the Nguyễn to submit to the authority of the court , and it was formally refused . In 1623 Trịnh Tùng died and was succeeded by his son Trịnh Trang . Now Trịnh Trang made a formal demand for submission , and again Nguyễn Phúc Nguyên refused . Finally in 1627 open warfare broke out between the Trịnh and the Nguyễn . For four months a large Trịnh army battled against the Nguyễn army but were unable to defeat them . The result of this war was that Vietnam had effectively been partitioned into northern and southern regions , with the Trịnh controlling most of the north and the Nguyễn controlling most of the south ; the dividing line was the Gianh River in Quảng Bình Province . This border was very close to the Seventeenth parallel ( in actuality the Bến Hải River located just to the south in Quảng Trị Province ) , which was imposed as the border between North Vietnam and South Vietnam during the Partition of Vietnam ( 1954 – 75 ) . While the Trịnh ruled over a much more populous territory , the Nguyễn had several advantages . First , they were on the defensive . Second , the Nguyễn were able to take advantage of their contacts with the Europeans , specifically the Portuguese , to purchase advanced European weapons and hire European military experts in fortifications . Third , the geography was favorable to them , as the flat land suitable for large organized armies is very narrow at this point of Vietnam ; the mountains nearly reach to the sea . After the first assault , the Nguyễn built two massive fortified lines which stretched a few miles from the sea to the hills . The walls were built north of Huế near the city of Đồng Hới . The Nguyễn defended these lines against numerous Trịnh offensives which lasted till 1672 . The story from this time is that the great military engineer was a Vietnamese general who was hired away from the Trịnh court by the Nguyễn . This man is given the credit in Vietnam for the successful design of the Nguyễn walls . Against the walls the Trịnh mustered an army of 100 , 000 men , 500 elephants , and 500 large ships ( Dupuy " Encyclopedia of Military History " pg . 596 ) . The initial attacks on the Nguyễn wall was unsuccessful . The attacks lasted for several years . Later campaigns . In 1633 the Trịnh tried an amphibious assault on the Nguyễn to get around the wall . The Trịnh fleet was defeated by the Nguyễn fleet at the battle of Nhat - Le . Around 1635 the Trịnh copied the Nguyễn and sought military aid from the Europeans . Trịnh Trang hired the Dutch to make cannons and ships for the Royal army . In 1642 - 43 , the Trịnh army attacked the Nguyễn walls . With the aid of the Dutch cannons , the Trịnh army broke through the first wall but failed to break through the second . At sea , the Trịnh , with their Dutch ships " Kievit " , " Nachtegaels " and " Woekende Book " were defeated by the Nguyễn fleet with their Portuguese ships . Trịnh Trang staged yet another offensive in 1648 but at the battle of Truong Duc , the Royal army was badly beaten by the Nguyễn . The new Lê king died around this time , perhaps as a result of the defeat . This now left the door open for the Nguyễn to finally go on the offensive . Nguyễn offensive . The Nguyễn launched their own invasion of Vietnam in 1653 . The Nguyễn army attacked north and defeated the weakened Royal army . Quảng Bình Province was captured . Then Hà Tĩnh Province fell to the Nguyễn army . In the following year , Trịnh Trang died as Nguyễn forces made attacks into Nghệ An Province . Under a new Trịnh Lord , the capable Trịnh Tạc , the Royal army attacked the Nguyễn army and defeated it . The Nguyễn were fatally weakened by a division between their two top generals who refused to cooperate with each other . In 1656 the Nguyễn army was driven back all the way to their original walls . Trịnh Tạc tried to break the walls of the Nguyễn in 1661 but this attack , like so many before it , failed to break through the walls . The end of the fight . In 1672 , the Trịnh army made a last effort to conquer the Nguyễn . The attacking army was under the command of Trịnh Tạc ' s son , Trịnh Căn , while the defending army was under the command of Nguyễn Phúc Tần ' s son Prince . The attack , like all the previous attacks on the Nguyễn walls , failed . This time the two sides agreed to a peace . With mediation supplied by the government of the Kangxi Emperor , the Trịnh and the Nguyễn finally agreed to end the fighting by making the Linh River the border between their lands ( 1673 ) . Although the Nguyễn nominally accepted the Lê King as the ruler of Vietnam , the reality was , the Nguyễn ruled the south , and the Trịnh ruled the north . This division continued for the next 100 years . The border between the Trịnh and the Nguyễn was strongly guarded but peaceful . Conquest of Huế - 1775 . The long peace came to an end in 1774 . At the time , the Nguyễn were under heavy assault from Tây Sơn army and part of their force had been defeated in recent fighting in Cambodia . The result was , the army in the north defending Huế was weak . Trịnh Sam , one of the last Trịnh Lords and ruler of the north , launched his attack on the Nguyễn on 15 November 1774 . For the first time , the Nguyễn walls were broken and captured . In February 1775 , the Nguyễn capital of Huế was captured by the Royal ( Trịnh ) army . After some fighting with the army of the Tây Sơn , a treaty was signed and the Trịnh army left the destruction of the Nguyễn to the southern rebels . A dozen years later the Trịnh Lords would be thrown out of Vietnam by the youngest and most popular of the Tây Sơn brothers . Heritages . To fight against the larger and more powerful Imperial army under Trịnh clan , Nguyễn lords actively expanded their territory to the South into the former land of Champa empire . They annexed much of the area that is now half of Vietnam . Both sides were enthusiastic in foreign technologies and welcoming to the outsiders . From foreign traders , craftsmen and mercenaries ( most from Portugal , Holland , China and Japan ) , both sides acquired advanced civil and military technologies such as mechanics , shipbuilding , artillery construction , as well as cultural and religious elements . This long period plays an important role in the formation of Vietnamese identity .