The Emerald Triangle

While the ‘Golden Triangle’ is well known both in Thailand and internationally, the ‘Emerald Triangle’ remains relatively unfamiliar.

This is where the Phu Chong Na Yoi National Park spans 686 sq km of rugged terrain, lies on the border of Thailand, Laos, and Cambodia. Once a strategic military zone, it is now a place of historical significance and, surprisingly, a tourism destination.

Yet, its past is largely unknown even to most Thais. The conflict that unfolded here between the Thai and Vietnamese armies, known as the Battle of Chong Bok, occurred between January 1985 and December 1987. It remains a largely hidden chapter of the Indochina War.

Following the 1978 Vietnamese invasion of Cambodia and the fall of the Khmer Rouge government in 1979, the Khmer Rouge forces fled to the border regions of Thailand, where they regrouped in the mountainous forests along the Thai-Cambodian border. With support from China, the Khmer Rouge continued to destabilise the region, targeting the pro-Hanoi People's Republic of Kampuchea.

Throughout the 1980s, the Thai and Vietnamese militaries clashed repeatedly, with the Vietnamese army conducting incursions into Thai territory in pursuit of Khmer Rouge guerrillas.

One of the fiercest battles during this period occurred at the Chong Bok Front. On the Cambodian side of the border, near Chong Bok, was a key Khmer Rouge stronghold called ‘Mountain 1001,’ where Khmer Rouge Defense Minister Son Sen had established a base against the Vietnamese.

After the Vietnamese army took control of Mountain 1001 in 1980, they clashed with Thai forces along the border. The Vietnamese army, occupying the area, claimed that it belonged to Cambodia, further complicating the territorial disputes. After the Vietnamese withdrew from Cambodia in 1989, Thailand took control of Chong Bok, and the territorial dispute with Cambodia continues to this day.

The Emerald Triangle lies about 125 km south of Ubon Ratchathani, at the meeting point of Thailand, Laos, and Cambodia. Visitors can reach the Thai Army’s Anupong Base, located on the scenic Phalan Suea Thon Bon Lake, by driving along Road 4121 from Nam Yuen. The base, named after Anupong Bunyaprathip who died during the Battle of Chong Bok is home to the Thai Border Guards (Thahan Phran), a paramilitary force.

Visitors need not worry about encountering hostile troops at the border. A friendly greeting might even result in a peaceful moment by the lake, where soldiers sometimes serve coffee to guests in typical Thai hospitality.

Just 1.5 km from the ‘Chong Bok Battlefield’ sign on the lakeside, the borders of Laos and Cambodia are separated by mountains, making it impossible to see them clearly. Yet, the harmony of the mountains, lakes, and skies offers a tranquil and beautiful view—an indigo world of serenity that can be enjoyed from a pavilion by the lake, with the gentle breeze completing the peaceful atmosphere.

After soaking in the view, a short drive of about 20 minutes along a narrow mountain road leads to Hill 500, the site of the most intense fighting during the Battle of Chong Bok. Hill 500 is part of the Dangrek Range, alongside Hill 631 and Hill 745. At the top of Hill 500 flies the Thai national flag, and below it stands a monument commemorating Sergeant Major Somchai Kaewpradit, a member of the 1st Infantry Division who died during a battle with the Vietnamese in June 1987. 

Despite the somber history, the sight of cartoonish soldier dolls holding mortars, rocket launchers, and rifles on a concrete platform brings an unexpected sense of lightness. The colorful, festive welcome signs appear out of place, resembling a Christmas display, but the presence of hidden bunkers and trenches in the surrounding bushes serves as a reminder that this was once a frontline of war.

The Communist Party of Vietnam, which claimed leadership over the Communist Parties of Indochina, viewed Thailand as a target for liberation after Laos and Cambodia. This fear of Vietnamese expansion continues to linger in Thai consciousness. In the 1980s, as Vietnamese forces pursued the Khmer Rouge into Thailand, the Thai government feared a conflict. While the Thai army occasionally engaged in limited combat with the Vietnamese, they also allowed incursions to go unchallenged in certain unmanned border areas. Chong Bok was one such location.

In March 1987, the Thai government publicly acknowledged the ongoing conflict at Chong Bok. The Vietnamese had occupied Chong Bok Hill with 1,500 troops, building a fortified base with tunnels and trenches. 

The Vietnamese army launched heavy artillery attacks against the Thai forces, which responded with fighter jets and ground troops, suffering 664 casualties. The battle continued until December 1987, with both sides claiming victory. It remains the only highland trench warfare battle on the Thai-Cambodian border.

Once the fighting ended, its history faded into obscurity. From Hill 500, if you look about 2 km ahead, the mountains on the left mark Champassak Province in Laos, while the right side leads to Preah Vihear Province in Cambodia. This area, known as the Emerald Triangle, is where the borders of the three countries converge.

In 2000, the governments of Thailand, Laos, and Cambodia launched the ‘Emerald Triangle Project’, aiming to develop the area for tourism and agricultural projects. The first proposal was to build a golf course and hotel complex with 27 holes, 9 from each country. The Thai government, dominant in the tourism sector, enthusiastically supported the plan.

However, environmental concerns about Phu Chong Na Yoi National Park, as well as territorial disputes between Thailand and Cambodia, soon emerged. By the mid-2000s, the project was abandoned. Thus, the name ‘Emerald Triangle’ was born, yet its dream of development remained unrealised.

The more pressing issue, however, is landmines. Over 100,000 mines were laid by the Vietnamese army in the 1980s. The Emerald Triangle Project, which sought to transform this land into a tourist destination, was conceived on a minefield. Worse still, in 2000, the Thai army added more mines to the area.

The scars of this ill-conceived project still remain in Chong Bok. The fields along the road and the path leading to Hill 500 are lined with landmine warning signs on every tree. The Thai-Cambodian border, particularly in Ubon Ratchathani Province, remains heavily mined, with Nam Yeun—located near Chong Bok—being one of the worst-hit areas.

Forty years since the end of the war, less than 20% of the mines have been cleared, and local civilians, soldiers, and villagers continue to fall victim to these hidden dangers.

As long as these mines remain, the war is not truly over. Anyone traveling through this historical region should be cautious.