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.