From the beginning of the Vietnam War, the NLF had used bases situated just inside the borders of neighbouring Cambodia. For many years US military advisers had wanted these bases to be bombed. President Lyndon B. Johnson had rejected this strategy as he feared it would undermine the anti-communist government of Prince Sihanouk.

Soon after becoming president, Richard Nixon gave permission for the bombing of Cambodia. In an effort to avoid international protest at this action, it was decided to keep information about these bombing raids hidden. Pilots were sworn to secrecy and their 'operational logs' were falsified.

The bombing failed to destroy the NLF bases and so in April, 1970, Nixon decided to send in troops to finish off the job. The invasion of Cambodia provoked a wave of demonstrations in the United States and in one of these, four students were killed when National guardsmen opened fire at Kent State University. In the days that followed, 450 colleges closed in protest against the killings.

The arrival of US marines in Cambodia also created hostility amongst the local population. The Cambodian communist movement, the Khmer Rouge, had received little support from the peasants before the United States invasion. Now they were in a position to appeal to their nationalist sentiments and claimed that Cambodia was about to be taken over by the United States. During 1970 and 1971, membership of the Khmer Rouge grew rapidly.

Laos, another country bordering Vietnam, was also invaded by US troops. As with Cambodia, this action increased the support for the communists (Pathet Lao) and by 1973, they controlled most of the country.