I am inclined to think, however, that what we are seeing is not a usurpation of power but a reflection of the fact that our Constitution simply does not tightly cabin the power to wage war. Even if we deny (and we should) that Presidents have authority to start wars on their own, the Supreme Court's decision in Bas v. Tingy in 1800 recognized that Congress can authorize the initiation of war by statute rather than by declaration of war. Here Congress certainly passed such a statute in the AUMF. So the immediate question is, "how much war did Congress authorize?"
One way to answer that question is to parse the words of the Authorization for Use of Military Force which Congress passed in 2001 to provide the legal basis for this war. But I want to approach the point from a somewhat different angle. Let's say that Congress only expressly authorized a limited war, targeted against Al Qaeda and its Taliban allies in Afghanistan. We fought that war, forced the Taliban from power, and largely drove Al Qaeda out of Afghanistan; let's say also that at that point, whenever it was, that war ended. But the end of that war didn't mean peace; on the contrary, that war flowed without interruption into the next one, the direct result of the first, in which we are fighting to establish a government in Afghanistan stable enough to withstand the assault of the Taliban and other Afghan rebel groups, and thus stable enough to keep Al Qaeda from returning in force to Afghanistan once again.
So when we crossed from the first war to the second, did our actions cease to be authorized by Congress? If peace had broken out, then when a new threat arose we might say that dealing with that threat required a fresh authorization. Perhaps if Al Qaeda had regrouped not across the border in Pakistan, but somewhere altogether different -- say, Malaysia -- then again we might say that pursuing Al Qaeda in this new location required a separate decision by Congress. But peace didn't break out; fighting never stopped; and Al Qaeda and the Taliban found refuge nearby in Pakistan, and the Taliban, at least, have used that refuge to pursue the war back in Afghanistan. To put the question starkly, then, did Congress authorize fighting the war and dealing with the aftermath, or only fighting the war?
War is full of perils, and we've always known this. I'm inclined to think that as a general proposition an authorization for war is an authorization to deal with the immediate conflict and with its aftermath. It would be possible to limit this idea by saying that the authorization extends only to the reasonably foreseeable aftermath, not to any and all terrible things that may grow out of a conflict -- but the trouble with this idea is that if anything is clear about war, it is that war's consequences are hard to foresee. It's also arguable that everything that has happened in Afghanistan was reasonably foreseeable, even if we didn't foresee it. Congress certainly could write an authorization that was very strictly limited -- but it's not likely to do that, because it too knows that war is full of unpredictable dangers. (As a case in point, the Authorization for Use of Military Force that began our war in Afghanistan uses language that, whatever its exact breadth, certainly is not "very strictly limited.")
Then is there any limit at all, in conflicts where peace does not ever arrive and fighting spreads but does not leap to altogether separate arenas? I wouldn't want to say no, but it isn't easy to see legal rules that mark out these boundaries. What is easier to say is that there are political limits: if and when Congress decides matters are out of hand, the Constitution clearly gives it power to act on that conviction. Doing so isn't easy, since Congress cannot legislate over the President's opposition unless it musters a supermajority of its members to support what it is doing -- and few members of Congress are eager to undercut a President while our troops are exposed to danger. And that's my point: once a war is authorized, I think the constitutional system we have does not make it easy either to mark out the limits of that war or to end a war that the President believes must continue.