President Bush last night:
Yet it would not be like us to leave our promises unkept, our friends abandoned, and our own security at risk. (Applause.) Ladies and gentlemen: On this day, at this hour, it is still within our power to shape the outcome of this battle. Let us find our resolve, and turn events toward victory.
In other words, we've invested so much, why walk away now? But it's a logical error, known in social science as the dreaded sunk costs fallacy. Here's a snippet from Why Hawks Win, Daniel Kahneman and Jonathan Renshon's article in the current issue of FP:
When things are going badly in a conflict, the aversion to cutting one's losses, often compounded by wishful thinking, is likely to dominate the calculus of the losing side. This brew of psychological factors tends to cause conflicts to endure long beyond the point where a reasonable observer would see the outcome as a near certainty. Many other factors pull in the same direction, notably the fact that for the leaders who have led their nation to the brink of defeat, the consequences of giving up will usually not be worse if the conflict is prolonged, even if they are worse for the citizens they lead.
U.S. policymakers faced this dilemma at many points in Vietnam and today in Iraq. To withdraw now is to accept a sure loss, and that option is deeply unattractive. The option of hanging on will therefore be relatively attractive, even if the chances of success are small and the cost of delaying failure is high.