It's more complex. The Communist Vietnamese originally backed the Khmer Rouge. Then they had a split and the Khmer Rouge attacked Vietnam border towns and com

It's more complex. The Communist Vietnamese originally backed the Khmer Rouge. Then they had a split and the Khmer Rouge attacked Vietnam border towns and committed some pretty horribly atrocities. The Vietnam brought down the Khmer Rouge, but it all started as an attempt to protect their own population.