A ceiling, or cap, on troop levels in Iraq would only be a means to an end, and the just the beginning of a process that would be useless, even harmful, if not carried forward to its end. So yes, let us have a Congressional cap, but only if we have a majority willing to follow the process that cap starts through to its logical conclusion.
I explore the process below the fold...
No form of Congressional cap on troop levels in Iraq would get near garnering a majority without an escape clause allowing the President to escalate as much as he wanted in "emergent contingencies" in which such escalation of force was deemed necessary to protect the troops. Sure, Congress could incorporate an escape clause to the escape clause, whereby, in the manner of the War Powers Act, the administration would have to justify the claimed necessity after the fact. So, Congress is going to substitute its opinion of non-necessity, and pull the extra troops back, for the opinions of the generals the administration will send to the hearing on justification, who will of course say that the escalation is necessary to protect the troops previously committed?
The only practical restraint on Presidential conduct of a war needs to either happen before the shooting starts, or involve getting a new CinC. The expert witnesses all work for the CinC, and their expert opinion about the safety of their troops is practically unassailable, in ways their opinions about the necessity for a given intervention in the first place, its likely outcome, and even the resources needed -- the big picture -- can never be. Deciding whether or not to go to war is a political decision, in which generals are no more expert than you or I, though their opinions about means and ways of accomplishing what we want to accomplish with a given war, are of course more authoritative than mine. But once the war is started, and our troops are in harm's way, all questions tend to devolve to the tactical, and we are at the mercy of the experts who take their marching orders from the CinC. If Congress should reach the point of conviction that the CinC is systematically tampering with these expert witnesses, there are no clearer grounds or stronger necessity for impeachment.
The position of CinC is essentially that of chief general/admiral, the one conduit the Constitution requires be followed when the nation gives marching orders to the military. Such a position was thought necessary because we were to have no king, and the king in the British system, upon which ours is modelled, fulfilled that need for unity of national command, though Parliament set policy by that time. The British had started appointing CinCs, the post held by a general, in wartime during the Seven Year's War, because even the early stages of that worldwide conflict showed the need for one central clearing-house to coordinate the actions of fleets and armies spread all over the globe, and the king's personal retinue was no longer suitable to the task (as the WH staff is so unsuited). The President's status as CinC is usually portrayed as something giving him immense authority in military affairs in particular, and foreign policy in general. But that is true only as long as Congress, which, no less than 18th Century Parliament, is the true Decider of policy in our political system, defers to the servant where it should assert a master's authority over him, and sit in judgment of his competence in discharge of his office. See the CinC in his proper light, as merely the chief general/admiral set atop our military chain of command, and you can see that this particular general/admiral can and should be relieved of his command when he proves incompetent at that command. Yes, the President is many things beyond his role as CinC, so the incompetence should be of such magnitude that the danger it presents to the republic outweighs the danger that changing Presidents presents to the republic. Personally, I believe that we have already passed this point, and I would vote to impeach and convict for dereliction of duty and incompetence in command as Commander in Chief, but that's just me. On the other hand, most people seem to agree with me on the incompetence in command question, so perhaps if our political and media elites would present the question to the people in its proper light, my opinion on impeachment would become the majority position. Practically speaking, it will probably be necessary to clarify the will of the people to get them top the point of understanding the need for impeachment by doing something novel -- forcing the system to play out the process openly, honestly and consistently.
So yes, by all means set a cap, and require retrospective justification for any of the discretionary escalation that will have to be allowed. But don't bother unless you fully intend to follow this process to its conclusion, and diligently question the assertions of the generals at any hearings on justification for this discretion, for any signs of coercion by the CinC. Be prepared to use credible claims of any coercion to impeach and convict the CinC, preferably before the sun sets on that very day. Don't expect generals to break with their CinC and resign on principal rather than collaborate in his mad schemes unless you are prepared to back them up, to sit in judgment, and remove the CinC if he is found less competent than the general he forced from office for being right. If you're not prepared to do all of these things, don't bother with the preliminaries. They will only make you look weak and infirm of purpose when this CinC calls your bluff, as he assuredly will. He doesn't seem to be good at much else, but he has proven to possess a knack for political poker. Don't do anything without looking four or five moves out, and being ready to respond to the likely countermoves and bluffs of our opponent.