Well, speaking as a Vietnam Vet, at least the troops know that up front (not that I agree with it). Many of us signed up for the Vietnam War in the belief that our country was committed to victory when JFK stated our county would "...bear any burden, pay any price, in the defense of liberty". I'm not complaining, I came back in relativey good condition. My anger and grief relates to those who lost their lives and those who continue to suffer.
Knowing it up front is worse than laboring under the illusion that we seeking to win. If we are on a glide path to get out of there in three years regardless, why be there is the first place? We need to set concrete objectives, i.e., the mission, and achieve it without setting artificial deadlines.