<P> On March 18, 1970, Sihanouk, who was out of the country on a state visit, was deposed by a vote of the National Assembly and replaced by General Lon Nol . Cambodia's ports were immediately closed to North Vietnamese military supplies, and the government demanded that PAVN / NLF forces be removed from the border areas within 72 hours . On March 29, 1970, the Vietnamese had taken matters into their own hands and launched an offensive against the Cambodian army . A force of North Vietnamese quickly overran large parts of eastern Cambodia reaching to within 15 miles (24 km) of Phnom Penh allowing their allies, the Chinese - supported Khmer Rouge to extend their power . Nixon ordered a military incursion into Cambodia by U.S. and ARVN troops in order to both destroy PAVN / NLF sanctuaries bordering South Vietnam and to buy time for the U.S. withdrawal . During the Cambodian Campaign, U.S. and ARVN forces discovered and removed or destroyed a huge logistical and intelligence haul in Cambodia . </P> <P> The incursion also sparked large - scale demonstrations on and closures of American college campuses . The expansion of the conflict into Cambodia was seen as an expansion of the conflict into yet another country, nullifying Nixon's promises of de-escalating the war . During the ensuing protests, four students were killed and a score were wounded by Ohio National Guardsmen during a demonstration at Kent State University . Two other students were killed at Jackson State University in Mississippi . In an effort to lessen opposition to the U.S. commitment, Nixon announced on October 12 that the U.S. would withdraw 40,000 more troops from Vietnam before Christmas . </P> <P> Following the coup, Sihanouk arrived in Beijing, where he established and headed a government in exile, throwing his substantial personal support behind the Khmer Rouge, the North Vietnamese, and the Laotian Pathet Lao . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (June 2012) (Learn how and when to remove this template message) </Td> </Tr> </Table>

When did the us first get involved in any way in vietnam