For the next 20 years the Trịnh and Nguyễn clans fought as allies against the usurper Mạc Đăng Dung to restore Đại Việt under rule of Lê Emperor.

Around 1530, Le dynasty loyalist rebels were forced into exile in Lan Xang (modern-day Laos) but they gathered a new army and captured some southern provinces of Dai Viet.

However, his nephew Trinh Tung became increasingly wary of Nguyen Hoang's influence in the court and kept Hoàng near him at all times for surveillance purposes.

Nguyen Hoang adopted Trinh Tung's two important generals as his nephews and incited them to rebel against their commander.

When the rebels stormed the court, Trinh Tung fled with the Emperor while Hoàng took to the Southern Provinces, never to return.

In 1600 the old Nguyen ruler broke relations with the Trinh court and titled himself as the Vuong (a term meaning prince or king).

[citation needed] In 1620, Nguyễn Phúc Nguyên officially refused to send taxes to the court in Hanoi.

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.

[citation needed] After the first assault, the Nguyễn built two massive fortified lines which stretched a few miles from the sea to the hills.

[1] 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.

Against the walls the Trịnh mustered an army of 100,000 men, 500 elephants, and 500 large ships (Dupuy "Encyclopedia of Military History" pg.

At sea, the Trịnh, with their Dutch ships Kievit, Nachtegaels and Woekende Book were destroyed in a humiliating defeat by the Nguyễn fleet with their Chinese style galleys.

[2][3][4][5][6] Trịnh Tráng staged yet another offensive in 1648 but at the battle of Truong Duc, the Royal army was badly beaten by the Nguyễn.

[7] The Trinh army lost 17,000 men within the first three days of the offensive against 25,000 Nguyen troops, which forced them to abort the attack and start peace talks.

[8] 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).

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.

[citation needed] 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 the Champa empire.

From foreign traders, craftsmen, and mercenaries (mostly from Portugal, Holland, China and Japan), both sides acquired advanced civil and military technologies such as mechanics, shipbuilding and artillery construction, as well as cultural and religious elements.

[citation needed] The Dutch brought Montagnard slaves they captured from Nguyễn lord territories in Quảng Nam Province to their colony in Taiwan.