The Administration's position on war in the gulf is that because Congress has authority to preclude it by passing a resolution to that affect, its failure to do so is tantamount to a declaration of war. However, this never was, and today it assuredly is not, a symmetrical situation. The fact that a majority of Congress can take action to stop a war if it can organize itself to do so is not remotely a functional substitute for the constitutional requirement that wars are not to be begun without Congress' affirmative approval.
The framers of the Constitution were explicit in their understanding that if Congress did not approve of our pursuance of a given war, it could end it by refusing further funding. But they also understood that once the President had committed "our boys" to the battlefield, it would become virtually impossible, emotionally and politically, to vote to cut off their "support." The Constitution requires a congressional declaration of war--it does not say that the President can commit troops to combat unless Congress takes steps to stop him--and there is every indication that this choice was deliberate.
This original decision has taken on additional validity today. The late 20th Century is in general an era of comparatively unassertive Congresses (perhaps one would better say Congresses comparatively unprepared to put their votes where their misgivings are). Today, members of Congress survive--and do they ever survive--by combining a maximum of individual services to constituents, and other interest groups seen as critical to reelection, with a minimum of actual legislative policy-making.
I do not wish to overstate: When the Constitution inescapably requires its assent, Congress remains capable of standing up to the President, as it did regarding the Bork nomination and the call for a constitutional flag-burning amendment. Decisive legislative action risks constituent support, though, and thus whenever there is any plausible way to avoid decision, Congress tends to take it. The most egregious example of this is probably the very subject under discussion, war-making. The Constitution tried to make that, too, a decision respecting which Congress' assent would be inescapably required. The system held for 161 years but broke down in 1950, over Korea--our clearest example, until now, of a war not authorized in advance by Congress--and it has stayed pretty much broken down ever since. (Indeed, in the Iraq case, unlike Korea, there can be nothing resembling a colorable presidential claim that the emergency came upon us so suddenly that there was no opportunity to seek authorization.)
Our lengthy, bloody war in Vietnam was authorized by Congress, most notably in the Tonkin Gulf Resolution, but with a maximum of obfuscation, disclaimers that the authorization being enacted certainly shouldn't be taken as indicating that those voting for it actually wanted the war to proceed.
And respecting our various smaller wars since--Grenada, Tripoli, Panama, the naval war with Iran and so forth--the President's confident assertions that such decisions are his alone, and the majority of Congress' unwillingness to take any action stronger than knitting their brows and waiting to see how the war in question plays politically, have both increased apace. For wars can go badly or wars can go well, and actually going on the record at the beginning (or for that matter any time before the end) can be risky. It is far safer to wait for the final curtain to decide whether you should applaud, or instead protest that you never really approved of the venture.
Actually the framers didn't want it to be symmetrical--but the asymmetry they sought was the exact opposite of that the Administration's argument seeks to establish. George Mason said that he was "for clogging rather than facilitating war; but for facilitating peace," and Oliver Ellsworth defended the requirement of congressional authorization by saying, "It should be more easy to get out of war, than into it."
Why should we care if Congress wants to surrender its constitutional prerogatives to the President? Doesn't it have ample means to get them back if it ever gets around to wanting to (or at least to make life miserable for any President who resists)? Probably it does, but the problem with the argument is that the war power wasn't given to Congress as a sort of "perk," but rather to ensure that no single person would be vested with the tragic decision to take the country into combat. Mason said he was "against giving the power of war to the Executive, because not safely to be trusted with it." (sic)
The prerogatives of congressmen aren't what's at stake here, but rather the lives of American (and other) young people--mainly, in this enlightened age of "volunteer" armies, the lives of young people whose disadvantage quota has already been filled.