In my opinion, Vietnam and Thailand are both equally territorial expansionists toward Cambodia and Laos. For one thing, Vietnam had a long history of divide and conquer. They would use all means of tactics to trick and entice Khmer leaders to follow their doctrine. I am not going to discuss the earlier history between the two countries. I will focus more on modern time especially starting from 1975. This is an exerpt from the Khmer Institute's article written by Dr. Gaffar Peang-Meth, a retired Political Science professor at the University of Guam.
According to Dr. Gaffar Peang-Meth, Vietnam had used the border tensions between Democratic Kampuchea and Vietnam as a pretext to a full scale invasion in 1979.
The Vietnamese southward expansion after Nam Viet freed itself in 939 from a thousand-year Chinese bondage was described by Vietnamese scholar Nguyen The Anh in "Le Nam Tien dans les textes Vietnamiens," as a centuries-long phenomenon called "Nam Tien" (progression southwards), "one of Vietnam's history's constants." Anh described the "sparsely populated and accessible land available for [Vietnamese] rice growers" to the south as "favorable for encroachment." Vietnamese "Confucian persuasion" was abandoned in favor of "an action resolutely imperialistic" to grab land and impose Vietnamese "administrative and cultural practice ... to better integrate [the new area] into the Vietnamese space." The migration was ongoing, even as other kingdoms were encountered. In 1406, the ancient kingdom of Champa's capital, Vijaya, was seized and the kingdom was extinguished in 1471. Then, in 1630, Vietnamese princess Ngoc Van, married to Khmer King Chey Chetha II, promoted Vietnamese settlements in the low delta Khmer territory of Preah Suakea (Ba Ria) and Prey Nokor (Saigon).
Historical records reveal that until the French protectorate was established in 1863, Cambodia was a battlefield for Thai and Vietnamese armies that fought on Khmer soil. Khmer dynastic quarrels led separate royal factions to seek support from Bangkok and Hue. Cambodia was known as a "two-headed bird" – a tributary state to both foreign capitals. In 1833, after Vietnam defeated the Thais in Cambodia, the bird head pointed toward Hue, and Vietnamization of Cambodia began in earnest: Vietnam installed teenager Ang Mey as queen, Cambodia's capital was renamed "Nam Viang," Cambodia's reorganization followed Vietnamese administrative lines, and authorities adopted Vietnamese names, customs and dress. In 1840, the Cambodian government was seated in Saigon, and Cambodia's name was changed to "Tran Tay" (western commandery).
On Christmas Eve 1978, more than 100,000 Vietnamese troops, backed by tanks and aircraft, crossed the border into Cambodia. In 14 days of fighting, Hanoi's army sent Pol Pot and his Khmer Rouge fleeing. The Vietnamese captured Phnom Penh Jan. 7, 1979, installed a puppet regime and stayed for the next 10 years. For victims of Pol Pot's genocidal rule, which began April 17, 1975 and resulted in the deaths of upwards of two million people, Jan.7, 1979 was the day of deliverance by Vietnam. Surely, Vietnam was their "savior" and their "liberator" at a time when the world watched and did nothing about the horrors of the Killing Fields. However, for many Cambodians, Jan. 7th is also a day of infamy. Pol Pot was replaced by those referred to as Cambodians with Khmer bodies but Vietnamese heads, the Khmer Viet Minh. This cohort was created by the Vietnamese Communist Lao Dong, trained at the Son Tay Military Academy and the Nguyen Ai Quoc political school, and led by a disgruntled regional field commander, Hun Sen, who became indebted to Hanoi for his return to power. Many Cambodians felt that substituting the Khmer Viet Minh for the Khmer Rouge was like replacing cholera with the plague.
The widely reported burning of homes and massacres of civilians in Vietnam's An Giang and Chau Doc provinces in 1977 by Pol Pot's guerrilla units offered an incitement to Vietnam, which was then busy strategizing and plotting Ho Chi Minh's grand design of a greater Vietnam. The Khmer Rouge’s belligerence gave the Vietnamese even more reason to put in play a takeover plan that would advance its goal of a federation of Vietnam, Cambodia and Laos.
What started as Nam Viet’s search for security and growth became a strategy for expansionism. The intention to expand its influence is illustrated even in the name of the political party founded in 1930 by Ho Chi Minh – the "Communist Party of Indochina." Ho did not just want to liberate Vietnam from the French; he defined the task of the CPI "to make Indochina completely independent."
Thailand is no difference than Vietnam when it comes to border expansion and annexation. Cambodian had lost 13 provinces to Thailand. They were Surin, Buriram, Srah Keo, Sisaket, Chantaburi, Trat, Nakhon Ratchasima, Ubon Ratchatani, Prachin Buri and Chonburi. Until recently, Battambang, Siem Reap and Posat were under Thailand control.
Thailand had sacked and looted Cambodia capital 3 times and taken priceless artifacts, treasures and educated teachers, dancers, royal courts to Thailand.
Hundreds of Thai troops were sent to occupy Wat Keo Sekha Kirisvarak pagoda in July, 2008. A group of 10 Thai soldiers who were camped at Wat Keo Sekha Kirisvarak pagoda have forbidden Khmer monks from constructing a residence building in the temple compound because they said that the area is a disputed zone. After this event, many armed clashed followed resulted in deaths and injuries on both sides.
Cambodia decided to have the International Court of Justice reinterpret the scope and meaning of the 1962 ICJ judgement. The court decided a temporary injunction to have a demilitary zone on both sides of the both worth 17 square miles. The final decision will be sometimes later on this year.
In conclusion, both Vietnam and Thailand will see to it that Cambodia will shrunk its size or ceased to exist as a sovereign nation. Vietnam will continue to promote Indochina Federation until Cambodia exstinct. Thailand will also continue to expand where they see the opportunity. Cambodia happened to exist between two land hungry neighbors.
Unless, Cambodia become stronger militarily, economically and politically, these two neighbor will continue to put a choke hold on her.