The Trinh and Nguyen fought the Mac together — then spent a century at war with each other

In 1527, Mac Dang Dung usurped the Le dynasty throne. The Trinh and Nguyen clans, both Le loyalists, fought side by side to restore the legitimate dynasty. Nguyen Kim, the senior commander, married his daughter to his subordinate Trinh Kiem, binding the two families by blood. For two decades they fought the Mac together.

In 1545, Nguyen Kim and his eldest son were assassinated by a Mac agent. Trinh Kiem took command of the royal army. To eliminate potential rivals, Trinh Kiem sent Nguyen Kim's remaining son Nguyen Hoang to govern the remote southern province of Thuan Hoa (modern central Vietnam). The move was meant to remove him from court politics — instead, it gave him a base.

Nguyen Hoang ruled the south for 55 years, expanding into Champa territory and building a separate administration. He continued to send taxes and troops to the Trinh for the war against the Mac. The Mac were finally defeated in 1592. But in 1600, Nguyen Hoang broke with the Trinh court, refusing to recognise the new Le emperor and taking the title 'Vuong' (prince). The south was now a separate state.

The Trinh-Nguyen War began in 1627 and lasted seven campaigns over 45 years. The Trinh had larger armies, but the Nguyen built two massive defensive walls at the Gianh River and armed themselves with Portuguese cannons. The Trinh enlisted the Dutch East India Company for ships and artillery. Neither side could break the other. The war ended in 1672 with Vietnam partitioned at the Gianh River, nominally under the Le dynasty but ruled by the Trinh in the north and the Nguyen in the south.