The 10-year-long Vietnamese invasion and occupation of Cambodia from 1978 to 1989 has remained a thorny issue to this day.

A Facebook post by Singapore Prime Minister Lee Hsien Loong that touched on the issue sparked rebuttals from ministers in both Vietnam and Cambodia, who insisted that the "invasion" was essentially the former liberating the latter from a genocidal regime.

However, historical facts have revealed a far more complex picture and a not-so-straightforward rationale behind Vietnam's decision to invade and occupy Cambodia.

Genocidal rule of the Khmer Rouge and their relations with Vietnam

Prior to Vietnam's invasion, Cambodia was reeling from the genocidal policies the Khmer Rouge regime put in place since their seizure of the capital city of Phnom Penh in April 1975, Time reported.

A so-called agrarian revolution resulted in the deaths of an estimated 1.7 million to 2 million Cambodians, including many ethnic minorities, Buddhists and Champa Muslims.

Additionally, in 1977, the Khmer Rouge government began carrying out bloody raids into Vietnam that often resulted in the massacre of thousands of Vietnamese, the Khmer Times said.

The Khmer Rouge justified the raids with their rhetoric of conquering lands that had historically been part of the Khmer empire.

China and the Soviet Union enter the scene

This was despite Vietnam having supported the Khmer Rouge up till 1974, at which point the Khmer Rouge purged Vietnamese-trained members from their ranks, National Interest reported.

National Interest further reported that the deteriorating relationship between Vietnam and Cambodia caught the eye of China, which was interested in cultivating ties with Cambodia.

In response, Vietnam signed a treaty of friendship with the Soviet Union which China opposed since the early 1960s, said former Deputy Prime Minister Wong Kan Seng in 2011.

Vietnam invades Cambodia over the raids

The Khmer Times cited the study of East German secret police files by German historian Bernd Schaefer, which said that Vietnam launched its invasion on Dec. 25, 1978 in response to the border raids.

Schaefer highlighted that the invasion had been timed to coincide with a period when China would be distracted over plans for relations to be normalised with the U.S. on Jan. 1, 1979, under Deng Xiaoping.

Within 13 days, the Khmer Rouge government was expelled from Phnom Penh, on Jan. 7, 1979.

As such, Schaefer added, while it was a fact that the Khmer Rouge regime was deposed by the Vietnamese, the invasion had not been undertaken purely on humanitarian grounds or to end the genocide perpetuated by the Khmer Rouge.

“They (Vietnam) did not primarily act because they wanted to end the genocide.

They wanted to get rid of an anti-Vietnam government, and put in a pro-Vietnam government. And in doing so, they got rid of the Khmer Rouge government. And that is a fact.”

Genocide condemnation becomes a means of legitimisation for puppet government

Vietnam subsequently established a puppet government under the pro-Vietnamese Communist Cambodian politician Heng Samrin.

Citing a book on the Cambodian genocide by historian David Chandler, Voices from S-21, Time reported that the new government based its legitimacy on the toppling of the Khmer Rouge.

As part of this legitimisation, the new government supposedly instituted Cambodia's Day of Remembrance on May 20, so as to help channel the "anger" of the Cambodians against the "genocidal clique" that had previously governed Cambodia.

At the time of its implementation in 1984, the Day of Remembrance was officially known as "The National Day of Hatred Against the Genocidal Pol Pot-Ieng Sary-Khieu Samphan clique and the Sihanouk-Son Sann Reactionary Groups".

Time further highlighted that throughout the 80s and 90s, the day would be marked by the burning of Pol Pot in effigy and the recounting of horror stories by survivors of the Khmer Rouge regime.

Presently, it was marked by re-enactments of the violence that was inflicted on Cambodians, as per The Star.

However, Chandler noted in his book, that the new Vietnamese-backed government was in no position to condemn the entire Khmer Rouge faction, given that many of its prominent figures were once from the Khmer Rouge themselves.

Vietnam finds itself isolated on the international stage

Meanwhile, Vietnam largely found itself isolated on the international stage.

Less than a week after Vietnam conquered Phnom Penh, then-Cambodian Prince Norodom Sihanouk took to the UN on Jan. 11, 1979, to condemn the invasion as "flagrant aggression", the New York Times reported.

His charge was echoed by China's UN representative, Chen Chu, which labelled Vietnam as "Hitlerite" and added that Vietnam was responsible for massive losses of life and the plunder of Cambodian towns.

In response, Vietnam's UN representative, Ha Van Lau, argued that it had a "sacred right" to defend its borders against Cambodian aggression.

Additionally, the atrocities of the Khmer Rouge regime made it "natural that such a regime should be overthrown" and that a new and better era had begun in Cambodia.

However, Vietnam found itself rebuffed diplomatically, in September 1979, when the UN voted to allow the representative of the ousted Khmer Rouge regime to keep its seat the General Assembly, instead of having him replaced with one from the puppet government, NYT reported.

U.S. imposed sanctions

The U.S. also imposed economic sanctions on Vietnam, including a trade embargo, and blocked financial assistance to Hanoi through the International Monetary Fund and the World Bank, LA Times further reported.

National Interest further reported that the U.S. also collaborated with Thailand and China to funnel arms to Cambodian factions that resisted the puppet government, with some of the weapons ending up in Khmer Rouge hands.

The upshot of this was that Vietnam soon found itself caught fighting a counterinsurgency war against such groups.

Cambodia begins to resent Vietnamese presence

Compounding the issue was the fact that despite being initially welcomed as liberators, the Cambodians eventually came to resent the Vietnamese as occupiers when they remained in the country for a decade until 1989, the BBC reported.

Schaefer highlighted that for much of their occupation, until around 1987-88, Vietnam ran Cambodia as a colony, with every decision having to go through the Vietnamese, the Khmer Times further reported.

"The Vietnamese  were making all the decisions. In the first years, when you wanted to meet a Kampuchean official, you could not meet him alone.

You had to have a Vietnamese minder sitting in on the meeting. The Vietnamese were saying, ‘if you want to give to them (the Cambodians), give to us first.’ The Vietnamese were very controlling.”

Al Jazeera further quoted Meas Nee, a Cambodian political analyst, who stated that few Cambodians have fond memories of Vietnamese occupation.

He added that many more Cambodians continued fleeing to refugee camps in Thailand, fearing the installation of yet another Communist government in the country.

Vietnam withdraws unconditionally on 1989

Subsequently, Vietnam announced on April 1989 that it would withdraw unconditionally from Cambodia in September of the same year, NYT reported.

Afterwards, Vietnam became one of the signatories to the 1991 Paris Peace Agreement which allowed for the formation of a UN Transitional Authority in Cambodia (UNTAC) in 1992.

It also enabled democratic elections in Cambodia in 1993, and injected money into Cambodia's failed economy, The Diplomat reported.

The BBC reported that Vietnam has since preferred to focus its narrative regarding the war in Cambodia on one aspect, a quick attack that toppled the Khmer Rouge, while not covering the 10-year occupation in equal depth.

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Top image from SJOBERG/AFP/Getty Images