Vietnamization was the Nixon administration's exit strategy from the Vietnam War. Initially called de-Americanization until the policy's chief architect, Secretary of Defence Melvin Laird, pointed out that it needed to be given a more positive name, the idea of the policy was that U.S. forces would be steadily withdrawn from South Vietnam while a massive programme of training and development for South Vietnam's own armed forces allowed them to step into the American role. It was hence a specific application of the Nixon Doctrine, which said that in the future the U.S. would not provide the manpower to defend its allies but merely support them in other ways.
Vietnamization was the product of a complex web of factors. After the Tet offensive in January 1968 had finally shattered the illusion that the U.S. military was making progress against Viet Cong guerrillas and their North Vietnamese backers, the pressures for peace talks and troop withdrawals in the U.S. became unavoidable. The Nixon administration tried to deftly get in front of the anti-war movement by announcing unilateral troop withdrawals, guaranteeing that American forces would be out of Vietnam on a particular timescale. Vietnamization was supposed to ensure that South Vietnam could continue to defend itself even after American forces had gone, and to satisfy domestic hawks that America's ally wasn't being merely abandoned.
However, the policy was really much more of a squaring of the domestic policy circle than it was a realistic way of winning the war. Laird backed the idea because he was finely attuned to the political mood, and also because he wanted to save money by withdrawing forces and then spend this money on modernizing America's armed forces; this is also how he managed to get the generals, understandably annoyed at being asked to essentially give up the war, to go along with it.
Henry Kissinger, the most powerful foreign policy figure in the administration, was sceptical that the policy could ever work. He simply didn't believe that enough South Vietnamese would want to fight and die for their own corrupt and narrow government. In his peace talks with the North Vietnamese, the latter's chief negotiator, Le Duc Tho, taunted him by asking how if the U.S. had been unable to prevail with half a million men and the South Vietnamese army it expected to do so without any U.S. forces at all. "It was a question," he later wrote, "which tormented me also".
Vietnamization had its ultimate test in battle. After a botched offensive into Laos which fatally undermined the South Vietnamese military's confidence, they survived an all-out blitzkrieg by North Vietnam in 1972 (the so-called Easter offensive) only with the support of a crushing blow against the Communists by U.S. air and naval power. This was sometimes presented as a victory for the South Vietnamese - and their forces had performed well on the ground - but their psychological reliance on American air support was much greater than was appreciated at the time.
By 1975, Congress had outlawed the use of air or naval support to South Vietnam, and they likewise refused to send military or economic aid to the country. Finally shorn of all American protection, South Vietnam fell to a crushing offensive. Vietnamization had worked in the short-term - some said to provide a "decent interval" between U.S. withdrawal and South Vietnamese collapse, so the abandonment didn't look too blatant - but proven incapable of creating a regime that could stand entirely independently.
Backers of Nixon claimed that had it not been for Watergate, he would have damned all restraints and launched another aerial defence of South Vietnam anyway. But Watergate did happen, and Gerald Ford was now president and was not so willing to continue pumping the poison of the Vietnam War through the American political bloodstream - and surely no policy so dependent on one man could have long succeeded anyway. Such was the determination of North Vietnam to unify Vietnam under Communist rule that the offensives were going to keep on coming - and America was always going to give up eventually. Vietnamization aside, it was only a matter of time.