the achievement of a military victory by U.S. forces in Vietnam was indeed a dangerous illusion
From what I understand McNamara did not quite say that. In his book "Argument without End" Mcnamara asked the question in Chapter 7 ‘Was military victory in Vietnam a dangerous illusion?’. Colonel Schandler (West Point graduate who served two combat tours in Vietnam, later took a PhD at Harvard in history, has written several books on Vietnam for the last ten or twenty years, has taught as a Professor of Military Strategy at the National Defense University) was asked by McNamara to write the chapter and in part he wrote,
‘The achievement of a military victory by US forces was indeed a dangerous illusion. At no time, beginning with the increase of US military advisers early in the Kennedy administration to the final withdrawal of American troops during the Nixon administration, would it have been possible at acceptable cost in terms of American and Vietnamese lives lost and without the risk of war with China or Russia to achieve a military victory in Vietnam.
The American failure in Vietnam was not a failure caused by the limitations placed upon military action. The American failure was caused by a lack of realisation that military power could not solve what was fundamentally a political problem. In the last analysis the US effort in Vietnam failed largely because it could not sufficiently revamp or adequately substitute for a South Vietnamese leadership, administration and armed forces inadequate to the task.’
As a participant, whose family paid dearly, I'm more inclined to the sentiments expressed by another,
The anti war movement in the US and the political pressure put on the politicians by the draft dodging peace loving crowd and the liberal media and their non support for the war is what ultimately brought down the US efforts to be successfull in South Vietnam
Many would argue that the military did win. Tet 68, for example, was played by the media as a humilitating defeat for the Allies when in fact it was a resounding win (militarily).