This was a major raid by Khmer Rouge forces over the Vietnamese border, resulting in the deaths of over 3000 Vietnamese.
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Was Pol Pot that foolish enough to think that he could carry out such a raid without a severe response from Vietnam? It turned out that this raid was a major catalyst leading to Vietnam’s invasion of Cambodia and subsequent overthrow of the Khmer Rouge.
The only logical explanation for this raid that I can think of is that Pol Pot was using Vietnamese border raids as a way of distracting the militant energy of his subordinates instead of letting this militancy result in a potential coup against himself. If this was the case, this got out of hand quickly. Or this was a decision made by a local Khemer Rouge commander that Pol Pot was not aware of.
In any case, this raid would later set a lot in motion two invasions (Vietnam vs Cambodia and China vs Vietnam) and years of insurgency in Cambodia.