Dao Duy Tu traveled throughout the Nhat Le River basin to study the terrain. The river collects water flowing from the mountains that is caught behind a large stretch of coastal sand dunes sixty kilometers long and, on average, five kilo- meters wide, oriented from southeast to northwest. The river flows north and enters the sea where the dunes narrow and end. Much of the basin is swampy and subject to inundation, but the northern third of it, where the mountain slopes are less than ten kilometers from the coast, comprises the historic choke point that had made this a borderland for centuries.
In 1630 and 1631, two walls were built across the northern part of the Nhat Le River basin. One wall reached the coast at the mouth of the river. It was initially known as “the wall of the teacher” (Truong Thay), in reference to Dao Duy Tu. Later generations called it the Wall of Dong Hoi, after the name of the city now located there. A second wall was roughly parallel to the first wall and around twelve kilometers further south. It was called the Wall of Truong Duc, which might be translated as “the long ‘heave ho’ wall,” bringing to mind that virtually the entire able-bodied male population of the southern realm was mobilized to construct these walls. These walls were around six meters high with cannon mounted every twelve to twenty meters. A third line of fortifications, called the Wall of Truong Sa, or “the long sand wall,” was built facing the sea along the crest of the dunes between the other two walls. An iron chain was prepared for stretching across the mouth of the Nhat Le River to block enemy ships in time of battle. Finally, the Dinh Muoi fortress was built in the middle of the rectangle formed by the walls and the mountains
. With this system of fortifications materializing, Nguyen Phuc Nguyen, taking Dao Duy Tu’s advice, sent an envoy to the north bearing a chest containing treasure and a hidden compartment in which was placed the royal edict and a coded message meaning “the appointment is not accepted.” Southern historians recorded a long interview that the envoy supposedly had with Trinh Trang in which he cleverly countered all the northern arguments against the south and after which he managed to get away back to the south before the secret compart- ment was discovered.
At this time, the death of Trinh Trang’s wife, Nguyen Ngoc Tu, daughter of Nguyen Hoang and sister of Nguyen Phuc Nguyen, removed a living link between the Trinh and Nguyen Phuc families. She had been an active presence in Dong Kinh society, patronizing Buddhism and building at least one temple. The death of Nguyen Phuc Nguyen’s wife occurred around the same time. She exemplified another significant family connection of that time. She was a daugh- ter of the renowned Mac prince and general Mac Kinh Dien. After the death of her father in 1580, she had been taken as a child to the south by an uncle, Mac Canh Huong. She was an aunt of Mac Kinh Khoan, who in the 1620s and 1630s led the Mac family and who, after his brief submission to the Le court in 1625, continued to harass Trinh Trang from his stronghold in Cao Bang.
Mac Canh Huong’s service in the entourage of Nguyen Hoang from the 1580s on suggests the complexity of Nguyen Hoang’s political calculations. Nguyen Hoang was an enemy of the Mac in his commitment to restore the Le dynasty, but he was an ally of the Mac against the ascendance of the Trinh. A son of Mac Canh Huong was married to a daughter of Nguyen Phuc Nguyen and had been adopted into the Nguyen Phuc lineage with the name Nguyen Phuc Vinh. He was a good general and led the army that successfully expelled the Cham invaders from Phu Yen in 1629. The Mac remained a worrisome but increasingly minor threat to the Trinh until the 1660s, capable of distracting raids whenever Trinh armies marched south. However, if some kind of implicit anti-Trinh alliance existed between the Mac and the Nguyen Phuc, conditions were never suffi- ciently favorable to enable it to materialize.
During the course of constructing the walls, Dao Duy Tu met a young military officer who so impressed him that he made him his son-in-law and introduced him to Nguyen Phuc Nguyen. This young man, Nguyen Huu Tien (1601–1666), was given an important command and became known for training his men to a remarkably high level of skill and discipline. He became one of the two southern generals to especially gain fame in the battles of the next thirty years. The other general was Nguyen Huu Dat, the young man who had called for war in 1623 and had participated in the battles of 1627.
The emplacement of so many cannons on the Nhat Le walls as well as the arming of ships with cannons required a large and reliable supply of artillery. In 1627, a Portuguese named Manuel Tavares Bocarro had established a gun foundry in Macau that for the next half-century produced cannons sold in many parts of Asia. Nguyen Phuc Nguyen purchased Bocarro cannons, but he needed more. In 1631, using methods learned from the Portuguese, a foundry for making guns and cannons was constructed near the walls, and the people of two villages were assigned to acquire the skills necessary for working there. One administrator, thirteen supervisors, and eighty-six workers were assigned to work at the foundry.
In 1632, Dao Duy Tu supervised a reform of procedures for mobilizing human and material resources that was ostensibly based on the late fifteenth-century model of King Le Tu Thanh, who had scheduled census registrations for taxation and conscription at six-year intervals and literary exams at three-year intervals. Population registers were to be kept up to date to facilitate a major military conscription every six years and a smaller mobilization during the intermediate third years. The country was organized into ten mobilization districts. Five of these were in Thuan Hoa, two were in modern Quang Nam, one in Quang Ngai, one in Binh Dinh, and one in Phu Yen. This may indicate the relative size of populations in these regions, but it surely indicates that the Thuan Hoa popula- tion was more militarized than those of areas further south. Three of the five districts in Thuan Hoa were in modern Quang Binh Province, namely the Nhat Le River basin and the territory north to the border. From this it is clear that the strategic border region was heavily populated with military settlements and may have provided around three-tenths of all conscripts
. Each district had a designated assembly point, and, during the years of major mobilizations, men with education could report to these places to take a one-day, two-part literary examination to measure ability in writing poetry and prose. Local magistrates gave a preliminary evaluation of these exams and judges in the Office of Commissioners decided the final results. The best candidates were exempted for five years (until the next large mobilization) from the head tax to allow them to continue their studies and to seek recommendations for govern- ment service. There was also an examination simply on ability to write classical characters, and from those who took this exam men were chosen to work as clerks in the “three offices.” If there were still vacancies to be filled in the “three offices,” they could be filled with men who contributed certain amounts of cash or rice and who presumably were in a position to assist in securing provisions for the military.
While some form of field tax was implemented to collect rice, this appears to have been done according to a relatively flexible method for supplying military units handled by the Offices of the Guard and of the Garrisons, for no specific information about this survives. It is probable that some rice was obtained by purchase rather than outright taxation. The main form of taxation was based upon the regularly updated population registers and was a head tax in cash levied on all male inhabitants. The head tax was based on the eight categories in which people were registered: active-duty soldiers, reserve soldiers, civilians, elderly (over 50 years old), disabled, mercenaries, indigent, and deserters. The tax rates were variable depending upon whether one was registered in Thuan Hoa or in the jurisdictions further south and whether one was classified as “primary,” meaning a full citizen, or as a “guest,” which apparently meant a relatively new settler who was still in the process of establishing a new life
. In general the head tax for “guests” was around one-half to two-thirds the amount levied on “primary” taxpayers. There was no tax on indigent or deserting “guests,” probably indicating that such people did not exist in any significant number if at all. It is a mystery how the tax levy recorded for deserters was enforced, unless it was treated as a payment allowing one to be registered out of that category. The rates for “guests” were a bit lower in Thuan Hoa than in the southern regions, perhaps indicating that refugees and settlers arriving from the north were encouraged to settle there. The rates for “primary” taxpay- ers show minor variations for the two jurisdictions, except for mercenaries who in the southern regions were taxed significantly higher than in Thuan Hoa and who, while constituting only one category in Thuan Hoa, were separated into three categories with different rates further south. This may indicate that “mer- cenaries” were more numerous in the south where military units were organized comprised of Chams, uplanders, and eventually Khmers and Chinese. Soldiers, both active duty and reserve, had the highest tax rates, indicating that, far from being a fiscal liability, military men and their families were integrated into the productive economy. The level of discriminating detail displayed in the head tax schedule suggests that the authorities were concerned to ensure that the rates were an appropriate and workable compromise between the government’s desire to maximize revenue and the taxpayer’s ability to pay.
Within a few years, Dao Duy Tu’s initiatives had affected nearly every aspect of Nguyen Phuc Nguyen’s government. He also successfully discouraged policies of which he did not approve. For example, in 1632, Nguyen Phuc Nguyen wanted to establish government monopolies for luxury goods such as pepper, bird nests, and other southern exotica. Such monopolies were standard in the north. Dao Duy Tu, however, felt that this would distract government with unnecessary opportunities for corruption and would stifle commerce that could more profitably be taxed. He made his point by appearing in court dressed as a merchant. Nguyen Phuc Nguyen reportedly understood his point and abandoned the idea
. In just a few years, Dao Duy Tu’s leadership had reorganized southern military and fiscal operations to channel human and material resources into a system of fortifications that protected the Nguyen Phuc realm from northern attacks during the remainder of the war. His unusual combination of abilities, unable to be exercised in the intellectually and socially constrained north, were appreci- ated and utilized in the south.