It wasn't destroyed; simply interrupted. COSVN (Central Office South Vietnam) in Cambodia was headquartered by both the NVA & VC. US ground troops crossed the border on 01 May 1970.
Laos was invaded in 1971 (Operation Lam Son 719); the US Army's final major offensive of the Vietnam War.
Cambodia was being invaded by both American and japenesse troops during their battles.
Vietnam and Cambodia
Richard Nixon
Believe it or not, the first troops into Vietnam were not conventional troops. They were actually the Green Berets, or the 10th Special Operations Forces. They were there to train the South Vietnamese to fight the Guerrilla Vietcong in South Vietnam. They first went in country in the 1950's.
Vietnam is a dense jungle that when the planes crashed, and/or the troops were captured it was easy for the Vietcong to relocate them without American troops finding them. There were tunnel systems used by the Vietcong to travel without being seen as well. Between those factors and the camps it was hard to locate all of our men. There is speculation of some of them being relocated out of Vietnam altogether upon capture as well.
Cambodia.
Vietcong forces used small engagements and ambushes to wear down American troops.
Vietcong forces used small engagements and ambushes to wear down American troops.
Vietcong forces used small engagements and ambushes to wear down American troops.
Cambodia was being invaded by both American and japenesse troops during their battles.
Not being able to find the Vietcong's. Vietcong's spent the day hiding, which restricted the American Troops attacks.
Vietnam and Cambodia
I'm not entirely sure what you are asking but yes there was a trail, called the Ho Chi Minh trail, that ran partly through Laos and Cambodia to transport supplies and vietcong troops to south Vietnam.
In the Vietnam Conflict, the US Military used primarily "Search and Destroy" tactics. These involved sending troops into an area with the hope of inducing an ambush or sending them in to destroy enemy camps.
Cambodia
Richard Nixon
Nixon resumed bombing of North Vietnam to instill a threat upon the North Vietnamese. He removed the troops and increased bombings leaving the North Vietnamese to believe only one thing... that he was going to drop an atomic bomb. Nixon nor any U.S. president would drop an atomic bomb threatening the lives of U.S. troops. Therefore, by bombing North Vietnam, he was able to continue to fight against the North Vietnamese without using his ground troops.