<P> In February 1965, a U.S. air base at Pleiku, in the Central Highlands of South Vietnam, was attacked twice by the NLF, resulting in the deaths of over a dozen U.S. personnel . These guerrilla attacks prompted the administration to order retaliatory air strikes against North Vietnam . </P> <P> Operation Rolling Thunder was the code name given to a sustained strategic bombing campaign targeted against the North by aircraft of the U.S. Air Force and Navy that was inaugurated on March 2, 1965 . Its original purpose was to bolster the morale of the South Vietnamese and to serve as a signaling device to Hanoi . U.S. airpower would act as a method of "strategic persuasion", deterring the North Vietnamese politically by the fear of continued or increased bombardment . Rolling Thunder gradually escalated in intensity, with aircraft striking only carefully selected targets . When that did not work, its goals were altered to destroying North Vietnam's will to fight by destroying the nation's industrial base, transportation network, and its (continually increasing) air defenses . After more than a million sorties were flown and three - quarters of a million tons of bombs were dropped, Rolling Thunder was ended on November 11, 1968 . </P> <P> Other aerial campaigns (Operation Barrel Roll, Operation Steel Tiger, Operation Tiger Hound, and Operation Commando Hunt) were directed to counter the flow of men and material down the PAVN logistical system that flowed from North Vietnam through southeastern Laos, and into South Vietnam known as the Ho Chi Minh Trail . </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>

American military build up in vietnam began under president