<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> <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> <P> In 1971 the U.S. authorized the ARVN to carry out an offensive operation aimed at cutting the Ho Chi Minh Trail in southeastern Laos . Besides attacking the PAVN logistical system (which would buy time for the U.S. withdrawal) the incursion would be a significant test of Vietnamization . Backed by U.S. air and artillery support (American troops were forbidden to enter Laos), the ARVN moved across the border along Route 9, utilizing the abandoned Marine outpost of Khe Sanh as a jumping - off point . At first, the incursion went well, but unlike the Cambodian operation of 1970, the PAVN decided to stand and fight, finally mustering around 60,000 men on the battlefield . </P>

When did america get involved in vietnam war