Mạc Đăng Dung
In 1520, fearing the ambition of Mạc Đăng Dung, the Nguyễn and the Trịnh left the capital Hanoi (then called Đông Đô) and fled south, taking the young new Emperor Lê Chiêu Tông "under their protection". This was the start of a civil war with Mạc Đăng Dung and his supporters on one side and the Trịnh/Nguyễn supporters on the other side. Thanh Hóa Province, the ancestral home to the Trịnh and the Nguyễn, was the battle ground between the two sides. After several years of warfare, Tông was assassinated in 1524 by Mạc Đăng Dung's supporters. A short time later, the resistance collapsed and both the Trịnh and the Nguyễn leaders were executed. However, this was just the end of the first phase of the civil war because in 1527 Mạc Đăng Dung usurped the throne. He killed his own puppet Emperor Lê Cung Hoàng and started a new dynasty, the Mạc dynasty. Within months the civil war broke out anew. Both the Trịnh and Nguyễn lords again took up arms in Thanh Hóa province and revolted against the Mạc. The leader of this second revolt was Nguyễn Kim. His daughter then married the new young leader of the Trịnh lord Trịnh Kiểm. Within five years, all of the region south of the Red River was under the control of the Nguyễn–Trịnh army but the two families were unable to conquer Ha Noi (known as "Thăng Long" at that time).