I'm tempted to think that the title of this thread says it all, but knowing the members of this board I'd most likely do well to add a bit more, eh? In any case, the Tet Offensive goes off more or less as planned during the Vietnam War, but for one reason or another North Vietnam and the Vietcong are not able to claim as much of a political victory as they do in OTL, or at all. Perhaps the assault on the Embassy fails even worse than it did in OTL, or Eddie Adams doesn't capture the execution of Nguyen Van Lem.
As a result, the Vietcong is crushed under the weight of American forces and the NVA faces interdiction by American forces as it attempts to replenish and resupply the Vietcong. At home, the Offensive is initially viewed as a shock, but when it is obvious that the American forces are getting the upper hand, political support for the war strengthens.
Does this have any effect on the Vietnam War (i.e., does it give South Vietnam and the US a chance of "winning"?), or does it merely postpone the inevitable?