Prince Norodom Sihanouk had proclaimed Cambodia neutral since 1955, but the communists used Cambodian soil as a base and Sihanouk tolerated their presence, because he wished to avoid being drawn into a wider regional conflict. Under pressure from Washington, however, he changed this policy in 1969. The Vietnamese communists were no longer welcome. President Nixon took the opportunity to launch a massive secret bombing campaign, called Operation Menu, against their sanctuaries along the Cambodia/Vietnam border.
This violated a long succession of pronouncements from Washington supporting Cambodian neutrality. Richard Nixon wrote to Prince Sihanouk in April 1969 assuring him that the United States respected "the sovereignty, neutrality and territorial integrity of the Kingdom of Cambodia." In 1970, Prince Sihanouk was deposed by his pro-American prime minister Lon Nol. The country's borders were closed, while U.S. forces and ARVN launched incursions into Cambodia to attack VPA/NLF bases and buy time for South Vietnam.
The invasion of Cambodia sparked nationwide U.S. protests. Four students were killed by National Guardsmen at Kent State University during a protest in Ohio, which provoked public outrage in the United States. The reaction to the incident by the Nixon administration was seen as callous and indifferent, providing additional impetus for the anti-war movement.
In 1971 the Pentagon Papers were leaked to The New York Times. The top-secret history of U.S. involvement in Vietnam, commissioned by the Department of Defense, detailed a long series of public deceptions. The Supreme Court ruled that its publication was legal.
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 secret war. After meeting resistance, ARVN forces retreated in a confused rout. They fled along roads littered with their own dead. When they ran out of fuel, 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 invading ARVN troops 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."
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.
Vietnamization was again tested by the Easter Offensive of 1972, a massive conventional invasion of South Vietnam. The VPA and NLF 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. But American airpower came to the rescue with 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 in August.