The Nguyen-Trinh Divide: A Civil War Subdivides

Conflict with the Mac had served to unite the Nguyen and Trinh clans, but it also became the catalyst for a rupture in their alliance as the two came increasingly to vie for supremacy.

Trinh begin to gain the upper hand after the death of the Nguyen family's paterfamilias. Nguyen Kim's assassination by a surrendering Mac general in 1545 paved the way for the Trinh to assert their dominance through a sustained effort to eliminate or sideline their ostensible partners. Trinh Kiem, the head of the Trinh clan, began to attack his rivals, killing at least one of Nguyen Kim's sons and raising the prospect of removing the others as well. The now precarious political climate in the north prompted a younger scion of the Nguyen clan, Nguyen Hoang, to request a posting to the southern reaches of the Vietnamese lands. The Trinh lord, seeing the advantageous prospect of relocating one of his rivals, readily granted the request, and Nguyen Hoang was sent to serve as the Le overseer for the frontier regions of Thuan Hoa and Quang Nam. Nguyen Hoang and a significant entourage departed for Thuan Hoa in 1558, thus temporarily relieving the tension that had built up between the two clans. This marked the beginnings of a separation of political authority in Vietnam, paving the way for conflict and warfare over the next two and a half centuries.

The territories of Quang Nam and Thuan Hoa had only recently been added to the Vietnamese realm, in the aftermath of the defeat of their Cham rulers by Emperor Le Thanh Tong in the 1470s. Vietnamese authority, however, remained largely nominal, and the two regions were only lightly settled by civilians and defended by garrison troops.

The state that the Nguyen began to carve out was one based on military authority rather than civilian control, and one tapping into the riches of the southern territories, derived substantially from the seaports once controlled by the Cham Empire. The Nguyen consolidated their authority and increased their economic strength sufficiently that when Nguyen Hoang was formally summoned back to the north in 1593, to assist in a final effort to overcome the Mac, he assented with little fear for his own safety. Between 1592 and 1600 the Nguyen leader participated in military operations with his erstwhile Trinh rivals in a joint project to decisively defeat the Mac and unify the northern region under Le (i.e. Trinh) governance. The effort largely succeeded, and the Mac were driven into the remote border mountains of Cao Bang. This refuge became a stronghold where the former regime's considerable military was able to protect the vestiges of a Mac state. The Mac were sheltered not only by the topography of the area but also by their cross-border Chinese neighbours, as the Ming court had a vested interest in undermining Vietnamese political unity. From their formidable perch, the Mac remnants continued to pose a periodic threat to the Le state, whose efforts to dislodge them repeatedly failed. The fall of the Ming dynasty (c. 1644) and the rise of the Qing marked the waning years of the Mac, and the Trinh took advantage of the Chinese dynastic transition to launch an aggressive campaign in the 1660s that drove the Mac into China, marking the final reincorporation of the border land under the control of Thang Long.

In helping to defeat the Mac, Nguyen Hoang recognised that he had contributed to consolidating Trinh political predominance in the northern region.

Over the next half a century from 1627 to 1672 the Nguyen and Trinh fought an episodic but ultimately inconclusive war. The Trinh state was much more populous, had larger armies, more military materiel and significantly more substantial natural resource reserves upon which to draw. The Nguyen state, by comparison, had only modest populations, limited local resource and a much less well equipped fighting force. Despite the imbalance between the two sides, the Nguyen were able to fend off repeated Trinh attempts to overwhelm their state. The southern state's shortcomings were offset by its highly defensible position (reinforced by a large-scale defensive wall), popula­tions already organised along military lines, and a combination of large numbers of war elephants and armaments produced by a Portuguese foundry at their capital in Phu Xuan. These assets enabled the Nguyen to fight the much stronger Trinh to a draw. Each side invaded the other, but both armies focused on taking prisoners rather than holding territory. The Nguyen in particular benefited from the violent capture of enemy soldiers and civilians, for these could be settled into frontier lands at the southern edges of their realm. Eventually a de facto ceasefire emerged in 1672 and the two sides were separated by what became a largely impermeable boundary at the Linh River.