Why was the Tet Offensive considered a "psychological" victory for the Viet Cong?
Whether and how much media reports influenced public perceptions of the American effort in Vietnam has been heavily debated. Journalists who covered the war do not agree on that issue.
To the extent that CBS television journalist Walter Cronkite was, in fact, an influential voice on the war, then there is a valid argument to be made that he and others in the media played an influential role in the perception of the outcome of the Tet Offensive as having been a major strategic defeat for the United States, despite the offensive's actual outcome.
Tet was a serious defeat for North Vietnam and its large guerrilla network in the South. The psychological impact of a sudden, surprise offensive waged by tens of thousands of Viet Cong guerrillas—an offensive that directly affected all of South Vietnam and even the grounds of the US Embassy in Saigon—was a more compelling story for journalists than the fact that the offensive was a resounding military defeat for those who waged it.
By the time the Tet Offensive was effectively crushed, the Viet Cong was destroyed as a cohesive guerrilla force. It suffered very serious casualties and was never again a major factor in the war’s outcome. Television reporting and, to a lesser extent, newspaper accounts, portrayed the offensive in an arguably misleading way. That explains the truism that the Tet Offensive was a military victory but a psychological defeat. If public perceptions regarding the American effort in Vietnam had already been turning against the war, a history of misleading press briefings and outright lies by the administration of President Lyndon Johnson regarding the war's course certainly contributed to the psychological impact of the Tet Offensive.
The Tet Offensive was considered to be a psychological victory for the Viet Cong and for North Vietnam even though it was a military defeat for them. The reason for this was that it demoralized the Americans and led them to lose faith in their leaders and in the war. It was soon after this that President Johnson decided not to run again for the presidency.
In the time before the Tet Offensive, the American political leaders had constantly been reassuring the country that the war was going well. They had been putting forth a narrative about how the war was just about won and about how the enemy was running out of the ability to mount any serious resistance. When the Tet Offensive opened up, it became clear that the US leaders had been wrong. The communists did not win the Tet Offensive militarily. However, they did prove conclusively that they were not finished as a force. Because they were able to prove this, they demoralized the Americans and helped turn American public opinion against the war. In this way, the offensive was an important psychological victory.