I read that Pol Pot threatened to invade Vietnam when he was in power. As far as I know, that was also a reason for the Cambodian-Vietnamese war. Did the regime really intend to invade Vietnam or were they just threats? .
What immediately comes to mind is that the Cambodian government was supported by the Khmer Rouge. Mao Zedong overthrew the Chinese government through guerrilla warfare. The Taliban overthrew the Afghan government twice. And the Spanish nationalists under Franco came to power through a civil war. Can you think of any more cases?
When the Khmer Rouge came to power, I mean.
The Khmer Rouge were nationalists who hated Vietnamese. Were they allies only because they had the same goal (to create a communist state)? According to English Wikipedia, they were allies in the Cambodian Civil War. https://en.wikipedia.org/wiki/Khmer_Rouge It's in the table at Allies. Later, the Cambodian-Vietnamese War broke out.
At least that's what it says here under currency. https://de.wikipedia.org/wiki/Democratic_Kampuchea