Before the Cambodian genocide Vietnam was in a war between the French.
Prince Sihanouk declared that he refuses to take side during the Vietnam war.
When Lon Nol took over, the US troops felt free to move into Cambodia, to deal with the Vietnam.
The Vietnam war erupted.
Vietnames communist had access to use the Cambodian port to ship in supplies needed.
The Vietnams hideouts during the war was in cambodia.
Cambodia became Vietnams war field.
The Vietnamese Killed over 750,000 Cambodians in suspecting them as north Vietnam suppliers.