Vietnam
Vietnam and Cambodia
Cambodia
Cambodia
Cambodia was officially neutral during the war.
Cambodia was neutral.
They were neutral.
Who told you it was, specifically how was Cambodia involved in the war? If you can boil down specifics, then maybe an explanation can be given.
The countries that were actually "physically" involved, and were bombed or had US aircraft crash into them, whether they were "officially" in the war or not, were: Lao's, Cambodia, North Vietnam and South Vietnam.
During the Vietnam War, the war spilled over to Cambodia.
Being so close to Vietnam, Cambodia was able to receive the "over-flow" from it.
The two countries have borders with Vietnam, and were a way to move men and materiel from North Vietnam to South Vietnam avoiding the narrow coastal region.
Vietnam
It didn't, Cambodia was officially neutral.
There are large tracts of forest in Cambodia, despite the deforestation campaigns waged on Cambodia during the American(vietnam)war. During the Vietnam War, there were many forests in Cambodia, Laos and South Vietnam. Only Airmen can report on forests for North Vietnam (they're the only ones that got to observe them).
Cambodia & Laos.
Communist control of Cambodia, Laos, and Vietnam.