<P> The ARVN launched Operation Lam Son 719 in February 1971, aimed at cutting the Ho Chi Minh trail in Laos . The ostensibly neutral Laos had long been the scene of a civil war, pitting the Laotian government backed by the US against the Pathet Lao and its North Vietnamese allies . After meeting resistance, ARVN forces retreated in a confused rout . They fled along roads littered with their own dead . When they exhausted fuel supplies, soldiers abandoned their vehicles and attempted to barge their way on to American helicopters sent to evacuate the wounded . Many ARVN soldiers clung to helicopter skids in a desperate attempt to save themselves . U.S. aircraft had to destroy abandoned equipment, including tanks, to prevent them from falling into enemy hands . Half of the ARVN troops involved in the operation were either captured or killed . The operation was a fiasco and represented a clear failure of Vietnamization . As Karnow noted "the blunders were monumental...The (South Vietnamese) government's top officers had been tutored by the Americans for ten or fifteen years, many at training schools in the United States, yet they had learned little ." </P> <P> In 1971 Australia and New Zealand withdrew their soldiers . The U.S. troop count was further reduced to 196,700, with a deadline to remove another 45,000 troops by February 1972 . As peace protests spread across the United States, disillusionment and ill - discipline grew in the ranks including increased drug use, "fragging" (the act of murdering the commander of a fighting unit) and desertions . </P> <P> Vietnamization was again tested by the Easter Offensive of 1972, a massive conventional NVA invasion of South Vietnam . The NVA and Viet Cong quickly overran the northern provinces and in coordination with other forces attacked from Cambodia, threatening to cut the country in half . U.S. troop withdrawals continued . American airpower responded, beginning Operation Linebacker, and the offensive was halted . However, it became clear that without American airpower South Vietnam could not survive . The last remaining American ground troops were withdrawn by the end of March 1973; U.S. naval and air forces remained in the Gulf of Tonkin, as well as Thailand and Guam . </P> <P> The war was the central issue of the 1972 U.S. presidential election . Nixon's opponent, George McGovern, campaigned on a platform of withdrawal from Vietnam . Nixon's National Security Advisor, Henry Kissinger, continued secret negotiations with North Vietnam's Lê Đức Thọ . In October 1972, they reached an agreement . </P>

When did the last american combat troops leave vietnam