Most of the unexploded ordinances in this country were dropped from big planes. Think clustered munitions and such. Those big planes came from a far away land.
https://quod.lib.umich.edu/t/tap/7977573.0003.107?view=text;...
US Air Force bombers like this B-52, shown releasing its payload over Vietnam, helped make Cambodia one of the most heavily bombed countries in history — perhaps the most heavily bombed.
To put 2,756,941 tons into perspective, the Allies dropped just over 2 million tons of bombs during all of World War II. Cambodia may be the most heavily bombed country in history.
https://gsp.yale.edu/sites/default/files/walrus_cambodiabomb...
That said, it seems the bigger problem in Cambodia is landmines, and AFAICT most of those were buried during Cambodia's internecine conflicts by various domestic factions and by the Vietnamese. See https://www.halotrust.org/where-we-work/south-asia/cambodia, https://en.wikipedia.org/wiki/Land_mines_in_Cambodia, and https://www.dw.com/en/clearing-cambodias-leftover-land-mines... According to that last article a large number of them were planted along Cambodia's western border by the Vietnamese to keep the Khmer Rouge at bay. See https://en.wikipedia.org/wiki/K5_Plan
It seems odd that the Vietnamese would expend much effort to keep the Khmer Rouge out of Thailand. Do you mean Vietnam's western border/Cambodia's eastern border?
For various reasons--human compassion, border control, protection of Vietnamese ethnic groups, geopolitical maneuvering, etc--Vietnam invaded Cambodia to put an end to the violent Khmer Rouge regime, installed a friendly government, and ended up in a protracted guerilla war w/ the Khmer Rouge, who would stage attacks from Thailand. In a way I guess it was sort of Vietnam's Vietnam.
It's been awhile since I read the political history, but IIRC Thailand tolerated the Khmer Rouge retreating into and camping along its border because Thailand and Vietnam were (and remain?) regional power rivals. They avoided direct conflict but were happy to poke a stick in the other's eye.
Yeah the loco Khmer Rouge repeatedly attacked Vietnam.
Vietnam was invading Cambodia, and trying to weed out the Khmer Rouge. The Khmer Rouge did what an inferior force does in this situation, and fought a guerilla war.