Regarding Assumptions and Afghanistan

Assume, arguendo, that the armed forces of the U.S. should be in Afghanistan. Use of the word "should" implies the existence of a good reason, so let's further assume there is one, e.g., their presence prevents the Taliban from dominating that nation, or their presence prevents the use of that nation as a staging ground for terrorist attacks.

Rather significant assumptions, some might say.

If they are made, though, the addition of 30,000 soldiers to that presence may make very good sense, if that addition is necessary to the continued success of the desired good outcome. Let's assume that's the case as well.

Adding 30,000 soldiers while telling the world that you will be withdrawing soldiers as of 2011 doesn't seem to make sense, even given the assumptions made, unless one makes further assumptions.  Those assumptions would be, I think, along the lines of (1) the danger of a Taliban resurgence or of the use of that nation as a haven for terrorists intent on wrecking havoc will diminish as of 2011, or (2) there will be a decreasing need for an American military presence to assure the desired good outcome commencing in 2011.

There would seem, though, to be little reason to believe either is likely to occur.  It would appear probable that the Taliban won't be leaving Afghanistan as of 2011, or that, if they should do so, they will return.  It would also seem probable that a steady decrease in the American military presence will not make the nation less a haven for terrorists.  It becomes more and more difficult to make assumptions on top of assumptions.

What then is the likely purpose of proclaiming that the U.S. will add 30,000 soldiers to its military presence and will begin withdrawing troops as of 2011?  It's difficult to think of any legitimate purpose.  As a result, it is natural to wonder if the purpose is not legitimate, or is an indication that those in power have no idea what to do, and are therefore willing to tread water, as it were, while they try to come up with a course of action.  Treading water in these circumstances, though, would seem to preclude adding soldiers; instead one would simply maintain the status quo.  So, does it make sense to infer that the 30,000 are being sent in an effort (vain, one would think) to placate political opposition?