I have previously described how North Korea sent fighter pilots to fly MiG-17s and MiG-21s in combat against American aircraft in North Vietnam, as well as teams of psychological warfare specialists to help the North Vietnamese conduct propaganda operations directed against South Korean troops serving in South Vietnam.

Now there is more to add to the story of North Korean participation in the Vietnam War. Newly available Vietnamese documents reveal that in 1965 North Korea dispatched an engineer regiment to North Vietnam to help the North Vietnamese construct a secret underground headquarters complex.

The underground headquarters was to be built in a remote mountainous region northwest of Hanoi for use by North Vietnam’s leaders in the event of an emergency, such as the destruction of North Vietnam’s headquarters facilities in Hanoi by US air attacks or an invasion of North Vietnam by US and South Vietnamese forces.

The secret headquarters was specifically designed to provide protection against both bombs and chemical weapons.  The construction of the underground headquarters complex began in October 1965, after the arrival of the North Korean engineer regiment. The North Korean engineers, who clearly had considerable experience in building such underground complexes, provided technical assistance, trained the Vietnamese to operate the equipment, and worked alongside the Vietnamese carrying out the construction of this complex and in assembling and installing “hundreds of tons” of equipment, including an air filtration system, in the headquarters complex. 

The North Koreans engineers worked on the project for two years, until the complex was finally completed in December 1967. 

Translations of several texts providing the details of such North Korean assistance to North Vietnam are appended below.

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Lê Thanh Nghị, 'Report on Meetings with Party Leaders of Eight Socialist Countries'," 1965.

[…]

People’s Democratic Republic of Korea:

Views Expressed by Comrade Kim Il Sung:

[…]

4/-With regard to aid, we [North Korea] have the following thoughts:

[…]

-Building a factory in a cave, such as a machinery factory, will require a cave with an area of almost 10,000 square meters. It took North Korea from 1951 to 1955-1956 to finish building its factories in man-made caves, but today we can do the work faster. Digging the caves and installing the machinery is difficult. You must install the machinery as you build the cave, constructing the factory in successive stages. It also took about three or four years, or two years at the very minimum, to build our underground headquarters command posts, but today we can build them faster. Using military personnel to build the caves is a very good practice. You can use simple methods, digging for a short time and building the projects half underground and half on the outside, following the method of building half outside and half in a cave inside a mountain. Then you can gradually, systematically, expand the project. We will immediately send 500 of our experts and laborers to work with Vietnamese cadres and workers. Aside from food and vegetables that Vietnam will provide, North Korea will provide all the other things that our workers need; our personnel will bring with them equipment and some supplies. If the construction work produces good results, we will send 500 more of our people. If they do good work, we will expand the construction work. We should begin the work on a small scale, perhaps three caves—one factory and two command posts—and then later we will expand the construction work. The work must be done quickly, it must be done well, and it must be completed as soon as possible, carrying out the work while at the same time conducting studies for more such projects. There is one important point, however, which is that when machinery is installed in caves and tunnels it can easily become rusty because of the high humidity; precision measuring devices can also be affected, so you must have air conditioners and de-humidifiers. North Korea will make machinery and tools to be used underground and will produce trucks for use underground. Building caves for aircraft (a regiment of 32 jet aircraft) is much more difficult, but we have good experience in this area, and our Chinese comrades who were sent here to learn how to do this have gone back home and have successfully built such caves. When building caves for aircraft you have to select an area that includes both mountains and flat ground. To summarize, if you want to build caves and tunnels, you have to assemble a work force, you have to have electricity, and you have to analyze the geology of the mountain and the nature of the rock that makes up the mountain.

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Nguyễn Văn Long et al, “History of the 299th Engineer Regiment, 1965-2005” [Lịch sử Trung đoàn Công binh 299, 1965-2005] (People’s Army Publishing House, Hanoi, 2005)

[pages 10-27]

[After describing the formation of the North Vietnamese Army’s new 299th Engineer Regiment in the summer of 1965]

…When it was first formed, the regiment was based in Trung Luong Village, Dinh Hoa District in what is now Thai Nguyen Province…On the morning of 5 August 1965 the regiment held a political-military conference attended by the regiment’s senior officers (military and political) from regiment headquarters down to the units…5 August 1965 became the 299th Engineer Regiment’s tradition day [official birthday]…As soon as the political-military conference ended, the regiment began feverishly preparing to carry out its central [main] mission, which was to build the Area B Project [Công trinh Khu B], which was the code name for the highest-level headquarters command post for our Party and our State [Government].  In order to carry out this mission, the first thing the regiment had to do was to quickly build a base camp and barracks for our allies (North Korean troops) and to build a network of roads within the construction project area. …

Because of the importance of this national defense project and because of the complexity of the technical design and construction work, the Party, State [Government], and Armed Forces of the People’s Democratic Republic of Korea [North Korea] would provide technical assistance and material supplies for the construction project, and it would send a construction regiment to help us to build the project. …The order to build the Area B Project was part of a network of construction projects to build secret heavily fortified installations to support the operations and activities of the highest-level agencies and organizations of our Party, our State [Government], and Armed Forces in the event that the enemy’s attacks on North Vietnam were escalated to the highest, most ferocious level.  Area B was located in the mountainous northern part of Thai Nguyen Province, which was very sparsely populated and where there were many rock mountains inside which the project could be built. Not only did this area contain ample sources of construction materials, such as bamboo, lumber, and rock, it was also an area where secrecy could be ensured so that the enemy would not detect our presence there and attack it.

When he gave the mission order for building this project to the 299th Regiment, Colonel Tran Ba Dang [Trần Bá Đặng], Deputy Commander of the Engineer Branch and personally provided instructions to the units that would be participating in this construction project, stressed that,

“The Area B Project, for which your regiment has been given responsibility, is the biggest construction project in the history of our nation. The entire project will be built in underground bunkers that must be able to withstand the explosive power of enemy bombs and to protect against the use of chemical weapons.  The existence of this facility must be kept absolutely, totally secret, both during its construction and during its use after it is completed.  This project will provide all the equipment required for a high-level headquarters to work and live. This means that the design, construction, and installation of equipment will require the highest level of technical skills.”

The regiment’s specific work assignment was to dig the tunnels and transport the dirt out of the tunnels and to provide support to the construction work, while the allied [North Korean] unit would provide us with technical assistance.

The regiment began the work on the project [began digging the tunnels] on 5 October 1965.  Working inside tunnels underground was totally new, not just to the regiment, but to the entire Engineer Branch at that time.  In order to ensure that the project was completed on schedule, the regiment began to dig three tunnel entrances all at the same time. Personnel from the regiment and allied [North Korean] personnel worked together on digging all three tunnel entrances.  To ensure secrecy, strict regulations were promulgated for carrying out this work.  Each component and each individual was only authorized to know about the specific task it, or he, was assigned to carry out, and the tasks had to be carried out to meet the most stringent technical standards in order to limit to the greatest extent possible any mistakes and accidents.  If just one tunnel section was built incorrectly and did not meet the required parameters and standards, the entire network would have to be repaired and corrected.  To carry out this totally new type of work, deciding that our personnel must study and learn as they worked, the Engineer Command directed the Project Office to work with the regiment to organize teams of officers and enlisted men who had good educational backgrounds and were able to learn quickly. These teams would work directly with the allies [the North Koreans] to resolve technical issues, such as surveying, drilling, setting off explosives, pouring concrete, and, of particular importance, dealing with situations that arose when large sections of the tunnels collapsed.  Because of our good organizational work, after a time of working with our [North Korean] allies, a number of our personnel were able to operate the different pieces of survey equipment, to determine directions and to find caves, and to solidly reinforce the roofs and floors of the tunnels and caves.  After studying and absorbing a number of advanced construction methods, within just a short period of time almost all of the regiment’s units and construction components increased their work productivity substantially. …In 1967, the second year of the Area B Construction Project began. This was the year that the project would have to be completed and placed into operation.  In addition to meeting the technical and aesthetic requirements, the requirement for the completion time was speeded up.  During this time the American imperialist attacks on North Vietnam became even more ferocious.  The enemy had bombed our important projects, such as the Haiphong Cement Factory, the Hanoi City Electrical Transformer Station, the Thai Nguyen Steel Mill, and a tremendous number of bridges and roads on our primary lines of communication with the goal of destroying North Vietnam’s economy and of blocking the flow of aid from North Vietnam to the war in South Vietnam.

To carry out instructions from the Engineer Command, the commanders of Work Site Z3 studied and digested the mission requirements of the different units, resolved to complete the mission on schedule while at the same time ensuring the quality of the work. At the same time, the commanders resolved to intensify ideological and organizational work and to come up with scientific, rational, and timely measures in order to constantly increase productivity and ensure absolute safety in order to complete the project as quickly as possible.

In order to carry out this resolution that had been approved by both the regiment and the work site, the battalions and work components organized work shifts so that the work could proceed continuously, day and night…At the 7th Battalion’s work area, Tunnel B1 was being dug through an section that was very difficult from a geological standpoint.  Because the limestone rock had many cracks and because of the high level of ground water absorbed into the limestone, the sides of the tunnel frequently collapsed, which caused the morale of quite a number of  our soldiers to waver as they feared being killed if an accident happened during the course of their work.  The requirements for the construction of this project gave the regiment no option other than to keep working and try to overcome the problem…In addition to working to reinforce the tunnel walls, the battalion and our [North Korean] allies conducted surveys, quickly determined the correct direction that the tunnel should take, and reinforced the floor and the roof of the tunnel so that concrete could be poured immediately in order to ensure the safety of our personnel. 

The pouring of the concrete was the most important task, the task of decisive importance to the quality of the project.  Concrete was poured as soon as each tunnel section was completed.  Steel frames were set up and welded together in sections, starting from the outside inward.  Only when the angles and the curve of the tunnel roof met the required specifications were the forms and reinforcements installed.

…In October 1967 the work on the main tunnels was finished.  Phase 2 of the project began - the installation of equipment inside the tunnels.  Allied [North Korean] personnel did most of the work of assembling and checking the operation of the equipment, while the regiment’s personnel focused on transporting equipment and machinery to support the work being done by the allies. …Because the excellent organization of the work assignments, after two months the officers and men of the 299th Engineer Regiment had safely and securely transported hundreds of tons of equipment and machinery into the tunnels and had helped our [North Korean] allies to assemble the equipment and machinery, thereby helping improve the quality of the operation of the Area B Project. 

During the entire time that they worked on the Area B Project alongside the 299th Engineer Regiment, the engineer soldiers of the Korean People’s Army truly sympathized with the conditions faced by our country and our people.  Despite all the difficulties, shortages, and hardships, they had carried out this hard, difficult, and very dangerous work day and night.

With their advanced scientific and technical skills and with their experience, the soldiers of the Korean People’s Army had installed the equipment and machinery for Project B precisely and safely, had tested and conducted trial runs of Project B’s equipment and machinery to ensure that everything met the technical requirements, and had also provided instruction and training that had helped the 299th Engineer Regiment to build up a core of technicians qualified to build underground tunnel and bunker projects.

Through this work the officers and enlisted men of the Korean People’s Army had helped to increase the solidarity between the Vietnamese and Korean peoples and had encouraged and motivated the officers and men of the 299th Engineer Regiment to be successful in building other such projects in the future. 

On 20 December 1967, after the electrical system for Tunnel B2 and the water supply pumping station were finished and operating, the Engineer Command conducted acceptance testing for the entire Area B Project.  After working for more than two years of work, the officers and men of the 299th Engineer Regiment had overcome all problems and difficulties and had successfully accomplished their assigned mission.  The regiment had dug 1,234 meters of tunnel into the mountain rocks and had helped our [North Korean] allies to install hundreds of tons of equipment and machinery while meeting all the technical and aesthetic requirements that had been set before work on the project had begun. 

On 24 February 1968, in accordance with Decision No. 876/TM from the Engineer Command, the 299th Engineer Regiment turned over the Area B Project, along with the regiment’s 9th Battalion and its 22nd Anti-Aircraft Artillery Company [Battery], to the 289th Engineer Regiment.…

***

Colonel Hoàng Viên et al, “History of Vietnam’s Military Engineers (1945-2005)” [Lịch Sử Công Binh Việt Nam (1945-2005)] (People’s Army Publishing House, Hanoi 2006)

[page 218]

…In accordance with agreements made between our country’s Party and Government and the Parties and Governments of the People’s Republic of China and of the People’s Democratic Republic of Korea, from 1964 to 1968 those two friendly countries sent a number of engineer units to our country to help us build national defense projects and airfields …