Bilateral relations between Cambodia and Thailand date to the 13th century during the Angkor Era. The Thai Ayutthaya Kingdom gradually displaced the declining Khmer Empire from the 14th century, French protectorateship separated Cambodia from modern Thailand at the turn of the 19th–20th centuries, and diplomatic relations between the modern states were established on 19 December 1950.
Relations between the two countries remain complicated. Incomplete demarcation of their boundaries has led to a protracted border conflict, most notably over the temple of Preah Vihear, which was brought to the International Court of Justice in 1962 but still saw military clashes emerge in 2008 and 2011. Cambodia's internal conflicts throughout the 1970s to 1980s often spilled over into Thailand, which received refugees but also gave indirect support to the Khmer Rouge led by the dictator Saloth Sâr (Pol Pot). Thailand now dominates over its poorer neighbour economically, and is the sixth largest investor in Cambodia.
Though relations are largely peaceful, the countries' peoples still bear a degree of enmity, and the two countries' shared cultural heritage has given rise to a fierce nationalist rivalry, often expressed through online flame wars over claims of ownership and origination of such heritage. These tensions have also erupted into violence when stoked by nationalist sentiments. Such episodes include a 2003 mob attack on the Thai Embassy in Phnom Penh, which prompted a downgrading of relations by Thailand. Thailand also downgraded relations in 2009 in response to the Cambodian government's support of former Thai Prime Minister Thaksin Shinawatra, while Cambodia terminated relations (represented by its embassy in Bangkok) twice in 1958 and 1961, during the Preah Vihear conflict.
In July 2025, violent clashes broke out between Thailand and Cambodia along their disputed border, resulting in over 30 deaths and the displacement of more than 270,000 people. 27 December 2025, both countries mutually agreed for ceasefire after long border disputes.
History Angkorian era
Early period The Thai or Siamese people have long been settled in the Khmer sphere of influence. The oldest evidence mentioning "Siam" is found in the Angkor Borei inscription, dated 1154 CE, which reads "Ku Syam" ("Siam [slave] woman") In the records of Zhou Daguan, a Chinese envoy who traveled to Angkor, the Siamese are noted as a separate group from the Khmer. The Khmer bought mulberry and silkworms from the Siamese because the Siamese knew how to weave clothes. The Khmer had to hire the Siamese to repair their clothes. There are also carvings of the "Siam Kuk" army on the walls of the Angkor Wat temple, with the inscription "Ne Syam Kuk" ("This Siam Kuk")ความเป็นมาของคำสยาม ไทย ลาว และขอม และลักษณะทางสังคมของชื่อชนชาติ, หน้า 30 and "Anak Rajakarya Bhak Paman Cheng Jhal Da Nam Syam Kuk" ("Military officials of Cheng Jhal city").
Following Jayavarman VII's death, the Khmer Empire experienced a gradual decline. Important factors were the neighboring peoples (Thai and Cham), chronic interdynastic strife, and the gradual deterioration of the complex irrigation system that had ensured rice surpluses. In the mid-13th century, the kingdom of Sukhothai was founded when a local Tai ruler declared independence from Khmer rule.Pletcher, K. (Ed.). (n.d.). Sukhothai kingdom. Britannica. https://www.britannica.com/place/Sukhothai-kingdom It was a small local power until Ramkhamhaeng inherited control of the kingdom in 1279. During the 13th century, Thai people began to rise up, Jayavarman VIII suffered a devastating war against the Sukhothai Kingdom led by Si Inthrathit. The Angkorian era existed until 1431, when the Thai under Borommarachathirat II, captured Angkor Thom and the Cambodian king fled to the southern part of the country.
Longvek era
During the Longvek period, due to the low populations of Southeast Asian states, human labor was the most important resource. Invasions to capture prisoners were common. Early 15th century Khmer kings led an army to attack Chanthaburi and Bang Khang, taking prisoners and bringing them back to Chatomukh.https://web.archive.org/web/20210420120825/https://cuir.car.chula.ac.th/dspace/bitstream/123456789/59148/1/5480517022.pdf?fbclid=IwAR0zi433uGbvqhcK7gN9vbE1Ar83j0LE0GiRs9U9cz4gY4T_7ffCPapYAlM Ponhea Yat also sent a raid to attack Chanthaburi. After the Ayutthaya capitulation to the Taungoo Dynasty, Baraminreachea and later Khmer kings launched many raids on Siam.
In relatiation for the attacks, King Naresuan of Ayutthaya ordered the invasion of Cambodia and captured many Khmer prisoners of war.
During the second fall of Ayutthaya, a large wave of Siamese refugees fled to Cambodia and the Principality of Hà Tiên.การเมืองไทยสมัยพระเจ้ากรุงธนบุรี, หน้า 174 Later, King Taksin of Thonburi led an army into Hà Tiên and Cambodia in 1771 to eliminate all the Ayutthaya princes, nobles, and officials who had fled as they posed as potential threats to legitimacy since Taksin was considered a usurper.
Udong era The establishment of Inner Cambodia, a region of primarily ethnic Khmers but ruled by Siamese lords brought many Siamese people to the region. Cities like Mongkol Borey, Serei Saophoan, Watthana Nakhon, Aranyaprathet, Battambang, and Siem Reap were settled by many Thais. Thai lords in the region built strong fortresses and city walls to consolidate the city.ศานติ ภักดีคำ. ยุทธมรรคา เส้นทางเดินทัพไทย-เขมร. กรุงเทพฯ : มติชน, 2557, หน้า 131-132
There were also Siamese theatrical troupes that crossed over to perform on the Khmer side. Many skilled male and female actors entered the service of the Khmer court, and some became drama teachers.เอนก นาวิกมูล. นักเดินทางชาวสยาม. กรุงเทพฯ : แสงดาว, 2562, หน้า 85–91 In a letter from Monsieur Wilmain to Monsieur Decourvière, it is stated that during 1785–1786, in the reign of King Rama I, Siam lost the war to Cambodia on January 18, leaving 500 Thai people and 16 converts stranded on the east bank of the Mekong River in Cambodian territory. In June of the same year, a number of Siamese soldiers deserted from Cambodia and returned to Bangkok."จดหมายเรื่องไทยส่งทัพไปเมืองญวน พม่ายกทัพมาเมืองไทยอีก จดหมายมองซิเออร์วิลแมง ถึงมองซิเออร์เดคูร์วีแยร์". ประชุมพงศาวดาร เล่ม 23, หน้า 188–190
However, a considerable number of Thais went into Cambodia voluntarily, for example, to work as officials, and many entered into service as royal consorts of the Cambodian king.http://digi.library.tu.ac.th/index/0098/30-10-Aug-2552/25PAGE98-PAGE109.pdf In the Siamese and Cambodian courts, there were kinship ties through marriage, especially in the inner court of King Norodom, who was always familiar with life in the Siamese court
French Protectorate of Cambodia After Cambodia became a point of power struggle between Thailand and Vietnam, King Norodom tried to prevent the two neighbors from taking over Cambodia by inviting France to make Cambodia its protectorate on 11 August 1863. Since then, Cambodia remained under France's rule until 9 November 1953.
World War II In 1941, Thailand invaded French Indochina. After a brief conflict, a treaty was signed that forced France to give the western parts of Cambodia to Thailand. In 1946, following Allied victory in WWII, Thailand was forced to return these territories to France.
Cold War In 1954, after French withdrawal, Thai military forces occupied the specific grounds of the Preah Vihear Temple in Cambodia and the surrounding "gray zones." They remained until a 1962 International Court of Justice (ICJ) ruling gave the temple to Cambodia.
During the 1970s, Khmer Rouge ruled Cambodia as Democratic Kampuchea carrying out the Cambodian genocide. This caused many Cambodian refugees fleeing to the Thai border to escape Pol Pot's regime.
On 1 January 1979, Vietnam invaded Kampuchea and removed Khmer Rouge from power replacing it with the Soviet-backed People's Republic of Kampuchea escalating into an eleven year war. Thailand refused to recognize the PRK and continued to support the deposed Democratic Kampuchea though Khmer Rouge and the two non-communist factions assembled the Kampuchean government-in-exile backed by the People's Republic of China, North Korea, ASEAN and the other Western powers. During that period, the Vietnamese soldiers attacked refugee camps near the Thai-Cambodian border over the course of its annual dry seasons.
By the time Communist regimes in Eastern Europe collapsed, Vietnam withdrew its troops from Cambodia in late 1989 leading up to the 1991 Paris Peace Accords which paved way for the restoration of monarchy in 1993. On 18 February 1994, Pol Pot and the Khmer Rouge had rebuilt their former jungle bases at the Dângrêk Range of Anlong Veng near the border with Thailand, which became the final headquarters for the Khmer Rouge members.
21st century In February 2025, Thai & Cambodian police raided a Poipet building, freeing 215 foreigners from multiple countries. Their various nationalities included 109 Thais, 50 Pakistanis, 48 Indians, 5 Taiwanese, and 3 Indonesians. This was the largest number of Thais ever freed from a suspected scam centre. South-East Asian scam centres have been operating for years, but faced increased scrutiny after a Chinese actor Wang Xing was lured to Thailand with a fake job offer, abducted, and then taken to a Myanmar scam center.
Conflicts and disputes 2003 riots
In January 2003, riots broke out in Phnom Penh after a Cambodian newspaper incorrectly reported that Thai actress Suvanant Kongying had stated Angkor Wat properly belonged to Thailand. On 29 January, the Thai embassy was burned, and hundreds of Thai immigrants fled the country to avoid the violence. Cambodians in Phnom Penh burned photos of King Bhumibol Adulyadej and Thais in Bangkok protested in front of the Cambodian embassy, burning Cambodian flags. This eventually led to the Thai government to sever diplomatic ties with Cambodia. Prime Minister Hun Sen banned Thai shows and films on TV stations.
2008 border disputes
The conflict between Cambodia and Thailand over land adjoining the site has led to periodic outbreaks of violence.
A military clash occurred in October 2008. In April 2009, 66 stones at the temple allegedly were damaged by Thai soldiers firing across the border. In February 2010, the Cambodian government filed a formal letter of complaint with Google Maps for depicting the natural watershed as the international border instead of the line shown on the 1907 French map used by the International Court of Justice in 1962.Cambodia Complains of Google Map Mistake at Preah Vihear Temple
In February 2011, when Thai officials were in Cambodia negotiating the dispute, Thai and Cambodian troops clashed, resulting in injuries and deaths on both sides. Artillery bombardment in the area occurred during the conflict. The Cambodian government has claimed that damage occurred to the temple.Petzet, Michael (2010). "Cambodia: Temple of Preah Vihear". In Christoph Machat, Michael Petzet and John Ziesemer (Eds.), Berlin: hendrik Bäßler verlag, 2010 However, a UNESCO mission to the site to determine the extent of the damage indicates that the destruction is a result of both Cambodian and Thai gunfire.