At the risk of wandering into Scott’s territory, it seems obvious to me that the Framers understood the dangers of executive power in wartime, and that they intendeded that Congress have significant powers to restrain the President. At the time of the writing of the Constitution, the Republic had been at war for well over half of its history, and this war was conducted, in large part, with a weak executive. Now, certainly the Constitution was designed in part to remedy the difficulties of engaging in international politics (including the fighting of wars) with a weak federal government, but the institutions established seem to have no particular preference for strong executive over strong legislative power in wartime. Indeed, it would seem quite the opposite…
I suppose that the next argument would be that the character of war and of international relations has changed so much over the last two hundred years that the burden of decision ought to shift, for wholly practical reasons, to the executive branch. Certainly, the conduct of foreign policy has become more complex since 1790, with the duties of even peacetime embassies and foreign stations dwarfing those of a hundred years ago. Warfare, and especially the support of the war-making machine, has also grown significantly more complex. It could be argued that this increase in complexity should result in the shifting of important decision-making from elected and appointed officials to career professionals in both the foreign policy and military services. Even if this is true, I’m not convinced that the trend should affect the balance of responsibility between the executive and legislative branches. The executive branch is every bit as susceptible to political pressure and political difficulty as the legislative, and the legislative has as much (and perhaps more) opportunity to accumulate and act on professional expertise as the executive.
It could also be argued that the tempo of international politics (and war) has increased to the degree that the normal legislative process is too cumbersome to operate properly in response to a foreign threat. It’s probably fair in the context of, say, a nuclear attack that the some important decisions need to be made by the executive without resort to immediate legislative input. But that only goes so far; most decisions, even in wartime, don’t require snap judgements (the invasions of Iraq or Afghanistan could certainly have been deliberated upon, as could the various decisions regarding the disposition of detainees), and, moreover, to say that the executive should be able to make some decisions without legislative input does not mean that it can make those decisions without legislative oversight and evaluation. It’s this, in particular, that Yoo and the Bush administration seem most unhappy about, but post hoc evaluation of executive decisions is absolutely critical to the functioning of a liberal democracy, and in particular to the construction of the institutions of governance in the United States.
The final point to be raised is that the assertion of increased executive power during wartime must be limited to some kind of time frame. In other words, even if you think that the gravity of decision-making shifts to the executive during war, there must be some mechanism for determining whether a state of war exists and when that state will cease. In the context of the War on Terror, which is essentially a conflict against a tactic, the war can, almost by definition, never end. It surely was not the intent of the Framers, and it surely is not defensible on its own merits, that the executive should be able to aggregate to itself emergency powers without any specific temporal limitation on those powers.