Trịnh Tráng
In 1620, after the enthronement of another figurehead Lê Emperor (Lê Than Tông), the new Nguyễn leader, Nguyễn Phúc Nguyên, refused to send tax money to the court in Đông Đô. In 1623, Trịnh Tung died, he was succeeded by his oldest son Trịnh Tráng. After five years of increasingly hostile talk, fighting broke out between the Trịnh and the Nguyễn in 1627. While the Trịnh ruled over much more populous territory, the Nguyễn had several advantages. First, they initially were on the defensive and rarely launched operations into the north. Second, the Nguyễn were able to take advantage of their contacts with the Europeans, specifically the Portuguese, to produce advanced cannons with the help of European engineers (for more details, see Artillery of the Nguyễn lords). Third, the geography was favorable to them, as the flat land suitable for large organized armies is very narrow at the border between the Nguyễn lands and the Trinh territories – the mountains nearly reach to the sea. After the first offensive was beaten off after four months of battle, the Nguyễn built two massive fortified lines that stretched a few miles from the sea to the hills. These walls were built north of Huế (between the Nhật Lệ River and the Sông Hương River). The walls were about 20 feet tall and seven miles long. The Nguyễn defended these lines against numerous Trịnh offensives that lasted (off and on) from 1631 till 1673, when Trịnh Tạc concluded a peace treaty with the Nguyễn Lord, Nguyễn Phúc Tần, dividing Vietnam between the two ruling families. This division continued for the next 100 years.