Trịnh Kiểm
The Mạc dynasty usurped the throne of the Lê family to create the Northern Court, whereupon descendant of the Lê kings was enthroned as "de jure" Southern court rulers by Nguyễn Kim. Shortly afterward, Nguyễn Kim, the leader of the Lê Dynasty loyalists and the "de facto" ruler of Vietnam, was poisoned by a Mạc Dynasty general. Kim's son-in-law, Trịnh Kiểm, took over the leadership and assassinated Kim's eldest son, Nguyễn Uông, in order to secure his authority. Nguyễn Hoàng, another son of Nguyễn Kim, feared having a fate like his brother Nguyễn Uông so he pretended to have mental illness and asked his sister Ngoc Bao, who was a wife of Trịnh Kiểm, to entreat Kiểm to allow Hoàng to govern Thuận Hóa, the southernmost region of Vietnam at this time. Because Mạc Dynasty loyalists were still occupying Thuận Hóa while Trịnh Kiểm was busy fending off Mạc forces in northern Vietnam during this time, Ngoc Bao's request was approved and Nguyễn Hoàng went south. After Hoàng pacified Thuận Hóa, he and his successor Nguyễn Phúc Nguyên secretly made this region loyal to the Nguyễn family; then they rose against the Trịnh Lords. Vietnam erupted into a new civil war between two "de facto" ruling families: the clan of the Nguyễn lords and the clan of the Trịnh lords. The Nguyễn lords continuously developed the territory and turned it into a strong base for their war against the Trịnh Lord and their expansion to the south. During this time, Thuận Hóa territory spanned from Quảng Bình to Thừa Thiên–Huế.