After 9/11/2001, President Bush declared war on "terror." Terror, and terrorist and terrorism, is an interesting idea - how does one define "terror?" How does one frame a war on terror? It's not a tangible enemy such as war against "Germany." In a situation such as the war on "terror," what we find ourselves fighting is an idea; a "terrorist" to one person is a "freedom fighter" to another, so it's sticky even to say who we are fighting against.
When one has such difficulty in defining what the enemy is, then it's no doubt going to be difficult determining how to define victory against that enemy. Here's where the concept of shifting goalposts comes to play. One the one hand, "victory" in war has traditionally meant the eradication of the enemy, or its unconditional surrender/capitulation. The latter cannot happen in a war against an idea, and the former is something that is only won when every member of that idea is defeated, which is an impossibility spawned by the definition.
This is the conundrum with the war on terror. You see, when we declared "war" on "terror," the Commander in Chief of our military stated that we would "conquer" the enemy; that we would win. Less than two years later, however, the President stated that the war on terror was not a war we could "win." Then, in 2005, the selfsame head of the Executive Branch stated that we would not settle for anything less than "complete victory."
Now, I'm not entirely sure how one can achieve "complete victory" in a war that we cannot win, but I would assume that it has something to do with shifting goalposts. These days, it's not about whether or not you "conquer" the enemy, especially if the "enemy" is whoever you say he is. Instead, the key is to redefine the parameters of the operation in such a manner that you can claim "victory" under the new rules (such as some are suggesting the NBA has done with referees in instances in the 2002 playoffs with the Lakers against the Kings and in 2005 with the Rockets against the Mavericks), or at least delay acknowledgement of the impossibility of reconciling the goal from the statement until such time as you can shift the blame for the inevitable failure on to someone else.
And people wonder why the President is being predicted to be remembered as a failure. When the defining piece of your legacy can be viewed as nothing less than a failure (and the absence of victory, however impossible to achieve, makes it exceedingly difficult to view it as otherwise), then that title is a difficult one to shake.