While it is true that no historical event exactly replicates another, it is certainly the case that what happened in Vietnam in 1972-1975 bears very closely on the current situation in Iraq.
To truncate the story drastically, what happened back then was the result of the correlation of four strategic factors:
(1) Hanoi's resolution to conquer the south. The North Vietnamese were held back by the failure of their spring offensive in 1972. That offensive was weakened by U.S. mining of the harbors and by the reluctance of China, in the swoon of the Nixon visit to Mao, to give full-bodied support to an invasion. But Hanoi simply bided its time.
(2) The withdrawal by the United States, ending in March 1973, of a combative military presence. Only a few hundred U.S. advisers were left in South Vietnam.
(3) The growing stability of the South Vietnamese government, which was assumed competent to carry out the terms of the Paris agreements of 1973. These agreements had been negotiated in dozens and dozens of meetings between Le Duc Tho and Henry Kissinger. The agreements called for the removal of U.S. forces, the cessation of North Vietnamese offensives, and recognition of the Saigon government as the ruling political entity in the south.
And (4) the progressive disunity of the United States government. Here we had the anti-war movement as a continuing force. But that movement attained dominance pari passu with the weakening of President Nixon. As Watergate metastasized from a "second-rate burglary" into grounds for the removal of a president, U.S. support for success in Vietnam wilted.
The parallels in the current situation are plain, beginning with the nature of the United States' participation. What we have right now is a progressively immobilized executive and a dissenting legislature, leading -- inevitably -- to an impotent military.
The question immediately posed is: Do we feel responsibility for what happens in the period ahead? The Iraqi government resembles the government of South Vietnam in 1973-'74 in that Baghdad is fighting, as Saigon fought, for a political system free of overweening foreign elements. But Saigon could not hold out in the long run without U.S. military support, and neither can Baghdad.