When Vietnam invaded Cambodia in 1979, most of the world (other than China) cheered them on; the removal of Pol Pot and the Khmer Rouge was seen as a good thing no matter who was doing it. However, the invasion had an unexpected side affect: the Khmer Rouge joined forces with their former rivals, the supporters of ousted prince Sihanouk, and fled to the western mountains of Cambodia to fight a guerrilla war against the occupiers. This led to tensions with Thailand when the guerrillas used the border area as a refuge. For a while, it appeared as if another invasion was imminent. The US guaranteed the safety of Thailand and rushed some military aid to them. In the end, Vietnam never quite brought themselves to do it, and they left the country in a negotiated settlement in 1992.
The POD here is that the guerrillas have a few spectacular successes, wiping out several large Vietnamese patrols, killing some of their higher commanders, and successfully raiding some of their bases. Thus, they make themselves enough of a danger that Vietnam takes the grim step of crossing the border in large numbers to destroy the guerrilla bases. Inevitably, they clash with the Thailand troops. Let's say this happens sometime between 1981 and 1984, during the worst of the crisis.
What next?