With the Iraq war going badly and a hostile Congress looking for the exit, comparisons to Vietnam are all the rage. Accounts of that war's endgame have generally been spun politically or distorted by hindsight, however.
Congressional anti-war activism, for example, was neither a heroic reining in of a runaway government (as the left claims) nor a perfidious stab in the back (as the right charges). It was simply the predictable epilogue to a drama that had largely played itself out years before. And while domestic politics established the broad guidelines within which different administrations operated, White House officials had substantial leeway to set policy as they wished. The real constraints, then as now, lay not in what was saleable at home but in what was achievable abroad.
From the start, the United States was fighting not to lose in Vietnam, rather than to win. In the 1960s, U.S. leaders believed that the fall of South Vietnam to communism would have terrible consequences, so they decided to prevent such an outcome by whatever means necessary. At first, this meant providing U.S. aid and advisers; then it meant facilitating the overthrow of President Ngo Dinh Diem; then, bombing North Vietnam; and, finally, sending U.S. ground troops to fight Communist forces directly. During the Kennedy and Johnson administrations, the toughest question—whether to accept the true costs of victory or defeat—was avoided. By gradually increasing the scale of the American effort, it was hoped, the Communists could be persuaded to cease and desist.
Once the patience of the American public wore thin, such an approach was no longer feasible. The Tet Offensive soured much of the establishment on the war and inclined them toward disengagement. Johnson himself, unwilling either to withdraw or to escalate, chose instead to renounce his re-election attempt, cap the war effort, and hunker down. He never accepted defeat, but the limits he set on American operations became political facts that restricted the choices available to his successor.
Coming into office in January 1969, Richard Nixon and his national security adviser, Henry Kissinger, understood that part of their mandate was to end the war in some way, and they wanted to do so for their own geopolitical reasons, as well. Still, they had no intention of "losing" the war outright or of abandoning South Vietnam under pressure from the enemy. So, they tried at first to achieve an old goal—an agreement formalizing simultaneous American and North Vietnamese military disengagement—with various new means, buying breathing space at home with token troop withdrawals. But the effort failed, and the war dragged on.
By late 1969, the Nixon team settled on a new approach combining gradual withdrawal, temporary protection of the regime of South Vietnamese President Nguyen Van Thieu, and intense diplomacy to enshrine these elements in a negotiated settlement. In the spring of 1969, there were almost 550,000 American troops in Vietnam. By the spring of 1970, there were over 400,000; by the fall of 1971, 180,000; by the spring of 1972, 65,000. The troop withdrawals undermined the Thieu regime's long-term security, but Washington took other measures (such as the Phoenix Program and assaults against Communist sanctuaries in Cambodia and Laos) to help protect it over the short term.
The administration did something similar at the negotiating table, formally protecting the Saigon government while ultimately agreeing to conditions that lowered its chances of survival. In September 1970, for example, Kissinger agreed that Northern troops could remain in the South after a settlement. Such a "cease-fire in place" would allow the Communist forces to renew their offensives with ease once the Americans had left; any agreement based on it would make an eventual Communist victory extremely likely. The North Vietnamese were unmoved, however, and persisted in demanding the one concession American leaders refused to make: a direct and immediate betrayal of Thieu and his government.
In the spring of 1972, the North Vietnamese launched a massive attack against the South (the "Easter Offensive") that gained ground at first but was eventually halted by South Vietnamese resistance, American tactical air support, and American strategic bombing. The Nixon administration's courting of the North's key patrons began to pay off, meanwhile, as Russia and China now sought to dampen the flames rather than fan them.
So, negotiations began to move forward, and by mid-October Kissinger and his North Vietnamese counterpart worked out a draft agreement that called for the removal of the remaining U.S. troops and the return of U.S. prisoners of war while deferring ultimate decisions about the South's political future. Kissinger knew it was the best deal available, but he also knew that it stacked the odds against the long-term survival of the Saigon regime. To bypass South Vietnamese objections, therefore, he decided to keep his negotiations secret until the last minute and then force Saigon to accept the final product.
But Thieu balked when presented with the fait accompli, pleading in tears for the Americans to hold out for better conditions. Nixon refused to force Thieu into line, so Kissinger had to tell the North Vietnamese that the signing of the agreement would be postponed. They retaliated by publicly revealing the deal (and the American commitment to it). This was the point at which Kissinger, desperate to keep the momentum moving forward, declared at a press conference that "peace is at hand."