Vietnam
Laos and Cambodia border Vietnam on the west. Vietnam is located on the eastern edge of the Indochina Peninsula in Southeast Asia.
Cambodia was invaded: Approximately 32,000 US troops, 21 miles, 60 day limit, 01 May 1970 to end of June 1970. Laos was invaded the following year, 1971; Operation Lam Son 719/Dewey Canyon II (US Army's last major campaign of the Vietnam War). Cambodia-Very successful Laos-Nearly a disaster
Northeast would be the former North Vietnam. N/W would be Thailand.
If they could it would have already happened
If South Vietnam fell, then it's next door neighbor Cambodia would fall, then Laos, then Thailand, then Burma, Pakistan, India, etc.
Cambodia had more bombs dropped on them than Laos during the Vietnam war. North Vietnamese solders would retreat into Cambodia and attack from there. Finally the US bombed North Vietnamese bases in Cambodia. They also established supply bases in Cambodia. Finally the US bombed the supply bases and supply lines into Cambodia.
The North Vietnamese Army would break the treaty and invaded South Vietnam. South Vietnam quickly fell to North Vietnam, which created one, Socialist Vietnam.
If someone said, "I was in Indochina", you would most likely reply, "what part?" Meaning Cambodia, or Laos, or South Vietnam, North Vietnam, etc. Vietnam is to Indochina as Texas is to America (or Texas is to the United States). Example: "What part of the US are you from?" Reply: "Texas"
Cambodia was caught in the middle. Powerful hostile NVA forces within their country; and a powerful US force wanting to destroy those NVA forces. A neutral Cambodia wanting no part of either, but unable to stop either one.
You would cross into Thailand and then Cambodia in order to reach Vietnam. However, certain land crossings in Burma / Myanmar are not permitted currently, although this may change in the future.
The US became involved in the Vietnam War in an attempt to stop the spread of Communism. The US worried that if South Vietnam fell, it would create a domino effect. The war lasted from 1955 to 1975.