The Vietnam War and the media "stab-in-the-back"

George Herring addressed the notion that the Vietnam War was lost because "the media stabbed us in the back", when victory was almost in the grasp of our intrepid generals, in his book America's Longest War: The United States and Vietnam, 1950-1975 (1986 edition):

The impact of public opinion on the decision-making process in March 1968 is difficult to measure. [Gen. William] Westmoreland and others have charged that a hostile and all-too-powerful media, especially the television networks, seized defeat from the jaws of victory by turning the public against the war and limiting the government's freedom of action just when the United States had a battered enemy on the ropes.  In fact, up to Tet, television coverage of the war tended overwhelmingly to be neutral or favorable to the government. The reporting during and after Tet was much more critical. A direct link between television reporting and public opinion cannot be established, however, and it seems more likely that the media's shift to a critical position reflected rather than caused the parallel shift in public opinion.  Vietnam was the first television war, to be sure, and it is possible that the nightly exposure to violence contributed to public war-weariness. Such an assertion can never be proven, however, and it can be argued as plausibly that television generated support for the war or even caused apathy.

In addition, the Johnson administration itself was at least partially responsible for media and public disillusionment during Tet. Its unduly optimistic pronouncements of 1967 made the shock of Tet greater than it might have been otherwise and widened an already large credibility gap. The President and his advisers could have corrected the distortions of the media, but their public response to Tet was itself halting and confused, in part because they were uncertain what was happening and how to respond. (my emphasis)

And he goes on to say that the notion underlying this stab-in-the-back argument, the idea that military victory was achievable at an acceptable cost, "remains quite doubtful."  He puts it politely.