This is a long but intriguing article that addresses this issue. Well worth a read.
"During the latter half of the 15-year American involvement in Viet Nam the media became the primary battlefield. Illusory events reported by the press as well as real events within the press corps were more decisive than the clash of arms or the contention of ideologies. For the first time in modern history, the outcome of a war was determined not on the battlefield, but on the printed page and, above all, on the television screen. Looking back coolly, I believe it can be said (surprising as it may still sound) that South Vietnamese and American forces actually won the limited military struggle. They virtually crushed the Viet Cong in the South, the “native” guerrillas who were directed, reinforced, and equipped from Hanoi; and thereafter they threw back the invasion by regular North Vietnamese divisions. None the less, the War was finally lost to the invaders after the US disengagement cause the political pressures built up by the media had made it quite impossible for Washington to maintain even the minimal material and moral support that would have enabled the Saigon regime to continue effective resistance."