Vietnamization
Nixon was elected President and began his policy of slow disengagement from the war. The goal was to gradually build up the South Vietnamese Army so that it could fight the war on its own. This policy became the cornerstone of the so-called "Nixon Doctrine". As applied to Vietnam, the doctrine was called "Vietnamization".
During this period, the United States conducted a gradual troop withdrawal from Vietnam. Nixon continued to use air power to bomb North Vietnam and Viet Cong forces in the south. The U.S. also attempted to disrupt North Vietnam's supply system by attacking the Ho Chi Minh Trail in the lead-up to withdrawal. The U.S. attacked North Vietnamese bases inside Cambodia, used its influence to achieve a change in government in Cambodia that led to the closing of Cambodian ports to North Vietnamese war supplies, and persuaded South Vietnam to launch a massive but ultimately unsuccessful operation into Laos to shut down the part of the Ho Chi Minh trail that traversed that country. More bombs were dropped on Vietnam under the Nixon presidency than under Johnson's, and U.S. casualties fell significantly. The Nixon administration was determined to remove U.S. troops from the theater while strengthening the ability of the ARVN to defend the south.
One of Nixon's main foreign policy goals had been the achievement of a breakthrough in U.S. relations with China and the Soviet Union. An avowed anti-communist early in his political career, Nixon could make diplomatic overtures to the Russian and Chinese communists without being accused of having communist sympathies. The result of those overtures was an era of détente that led to nuclear arms reductions in the U.S. and Soviet Union and the beginnings of a dialogue with China. In this context, Nixon viewed the Vietnam War as simply another limited conflict forming part of a bigger tapestry of superpower relations; however, he was still doggedly determined to preserve South Vietnam until such times as he could not be blamed for what he saw as its inevitable collapse (a "decent interval"). To this end he and his National Security Advisor Henry Kissinger employed the Chinese and Soviet foreign policy gambits to successfully defuse some of the anti-war opposition at home and secured movement at the negotiating table in Paris.
China and the U.S.S.R. had been the principal backers of the Vietnam People's Army through large amounts of military and financial support. The two communist powers competed with one another to prove their "fraternal socialist links" with the communist regime in the north. That support would increase in the years leading up to the U.S. departure in 1973, enabling the North Vietnamese to mount a full-scale conventional war against the south, complete with tanks, upgraded jet fighters, and a modern fuel pipeline snaking through parts of Laos and North Vietnam.
Nixon was roundly criticized for his heavy bombing of Hanoi in December 1972, which was partly facilitated by his diplomatic overtures to China, and which followed a breakdown in the Paris peace talks after peace appeared close at hand. Agreement had been reached in October 1972 between Kissinger and chief North Vietnamese negotiator Le Duc Tho, but the agreement was rejected by South Vietnamese President Thieu, who demanded dozens of changes to the text, including the critical demand that North Vietnamese troops had to withdraw from South Vietnam. The North Vietnamese rejected this, and then countered with many changes of their own. Nixon responded with Operation Linebacker II, which was condemned by one journalist as "war by tantrum." The bombing of Hanoi did, however, pressure North Vietnam back to the negotiating table, allowing America, and Nixon, a face-saving, or "decent interval", exit
