I gave an answer to a similar question not long ago that was more directed at the generalities and background of ‘Cambodia VS Vietnam’, that can be found here. As for this question, I think it could be more easily answered if it is broken into two sections:
Why did Vietnam invade Cambodia?
To what extent was this due to Human Rights violations?
As the other answer I linked gives a general overview of the long history of antagonism between the Khmer and Vietnamese, I will skip that part and simply refer to the two groups as ‘hereditary enemies’, with the Khmer playing the more ‘inferior’ role in this often violent dynamic. So, why did the Vietnamese invade Cambodia in late 1978? Well the most straightforward explanation is that it was in retaliation to Khmer Rouge incursions into Vietnam. Naturally the next question is, well... ‘why where the Khmer Rouge doing that?’ That is where the story becomes a little less easy to explain. Both the Cambodians and the Vietnamese achieved communist victories within weeks of each other in 1975. Soon after these governments stopped celebrating their respective victories and patting their communist ally’s backs it became clear that there would be an uneasy relationship between the two. Disputes over land and sea borders were almost immediately brought up, as were minor skirmishes between each army. These skirmishes and border raids – often involving the slaughter of civilians as Khmer Rouge troops travelled into Vietnamese territory – were not always unprovoked, and the case could be made from the viewpoint of the Khmer that the Vietnamese were looking at claiming more Cambodian land (as they had done since the middle ages).
In 1977 these clashes became more pronounced and it seemed as though the two former communist allies were falling into a war that both would perhaps had rather avoided. Journalist Philip Short explains this situation; ‘ill-founded or not, Cambodian fears were real. After two years in which both sides had tried to avoid a collision – the Cambodians because they wanted time to make their regime stronger, the Vietnamese because they expected to achieve their ends by political means – all their ancient hatreds abruptly reignited … the only choice in Pol Pot’s view, was what Douglas Pike termed the ‘bristly dog gambit’.
This metaphor is an attempt to explain why the Khmer Rouge, seemingly not at all equipped for this kind of conflict, were pursuing this policy. This apparently irrational behaviour could be seen in the same way that a small dog, surrounded by bigger, stronger dogs, can bristle and assume an aggressive posture and appear so fearfully troublesome, so indifferent to consequences, as to convince others to leave well alone. He would go on to say that ‘the gambit may not work, but it holds as much promise to the Cambodians as any other.’
Hanoi’s response to these incursions included bombing raids on Cambodian border positions and attempts at political negotiations. On the wider international-political side of things, we need to talk about who is on what team in this scenario in this point in time. For Cambodia, this was relatively straightforward: the CPK relied on China, and Beijing saw them as a barrier to the spread of Vietnamese power (read as Vietnamese/Soviet power). Vietnam had angered its former benefactor in China by siding with Moscow in the greater Sino-Soviet split, and was not in a particularly good position to ask for China to reel in the Khmer Rouge.
In late 1977 Vietnam retaliations involved 50,000 troops being sent onto Cambodian soil, something that the CPK could claim fully justified their fears that the Vietnamese had expansionist ideas for Indochina. These forces were eventually recalled, but the conflict was now fully out in the open. The CPK spent the next year turning further in on itself with hundreds of thousands purged, particularly in the Eastern Zones that bordered Vietnam. Former cadre sent to prisons such as S-21 were forced to confess their plans of subterfuge and collusion with the Vietnamese under torture, only reinforcing Pol Pot’s notions that there was a plot to overthrow him and it would come from the East.
Meanwhile, Vietnam began to be seen by China as fully committing to the Soviet bloc, and seen as the gateway for the potential of an Indochinese federation that was loyal to the Soviet’s rather than China; naturally something they would be increasingly weary of. The Vietnamese, unable to simply do nothing in the face of Khmer aggression (even calls for genocide of the Vietnamese) began actively cultivating a Khmer resistance to the CPK (thousands of Khmer Rouge cadre fled to Vietnam in the wake of purges initiated by Pol Pot) and they began planning their invasion of Cambodia to topple the regime and implant one that was friendly to Hanoi. This occurred on Christmas Day, 1978.
This is the bit where it can come down to what you think about what happened. How much of the Vietnamese invasion that I just talked about can be attributed to a concern about human rights violations?
Did the Vietnamese perform a humanitarian intervention in Democratic Kampuchea?
That depends on your point of view and I cannot give a definitive answer.
My opinion on the matter is that no, it was not. Human Rights violations were not a primary concern for Hanoi in the invasion of Cambodia, and certainly not the reason that the Vietnamese occupied Cambodia for a subsequent decade following the ousting of the CPK.
What this hinges on, and why I take this point of view, is that it certainly had the effect of a humanitarian intervention. But this was not the motivation.
When testifying for the Khmer Rouge tribunal, Stephen Morris saw this as the outcome - echoing other historians such as Chandler and Short. Morris went further however, and basing this on research in Soviet archives, claimed that the Vietnamese did intend to create an Indochina Federation that was a unified communist bloc. Whether this is true or not is up for debate, but it does add to the claims that the Vietnamese did not set out to simply help the people of Cambodia. They, like any country, we’re self interested in their invasion and subsequent occupation of Cambodia.
I am not saying this was Vietnamese aggression, the case can certainly be made that it was self-defence, but how much it can be called a humanitarian intervention is not settled. Was the US invasion of Iraq that removed a terrible dictator a humanitarian intervention? Could be apples and oranges there but again the argument can be made that it had that effect, considering how terrible the respective leaders of Iraq and Democratic Kampuchea were.
For example, Gary Clintworth’s ‘Vietnam’s intervention in Cambodia in International Law’, argues the case that it can and should be considered as such. Others are less convinced of the altruistic intent of the invasion. For instance, Philip Short in ‘Pol Pot’ writes:
“To the overwhelming majority of Cambodians in January 1979, the Vietnamese appeared as saviours. Hereditary enemies or not, Khmer Rouge rule had been so unspeakably awful that anything else had to be better. Vietnamese propagandists exploited this to the full. Vietnam’s army, they claimed, had entered Cambodia not to occupy it but to deliver the population from enslavement by a fascist, tyrannical regime which enforced genocidal policies through massacres and starvation. That was of course untrue. The Vietnamese leaders had not been bothered in the least by Khmer Rouge atrocities until they decided that Pol Pot’s regime was a threat to their own national interests.”
This is emphasised by the actions of the Vietnamese in the spring of 1979 when Phnom Penh was systematically looted and aid which was eventually delivered to the Cambodians by international organisations was also taken, in part, by Vietnam.
This answer should not be misconstrued as a defence of the Khmer Rouge, nor a condemnation of the Vietnamese. I am thankful and glad that the invasion occurred, as were the millions that were able to survive the torment and nightmarish conditions of Democratic Kampuchea.
That being said, I think it is wise to assume a level of political realism in regard to the Vietnamese invasion of Cambodia that incorporates their national interest being the first and foremost goal.