Jack Goldsmith does not think they were a good idea, but thinks they are constitutional and fall within the limits of presidential military power as defined over time by the courts:
I do not believe that the military action in Libya is unconstitutional.
Legal scholars disagree about the original meaning of the Constitution's conferral on Congress of the power "to declare war." Many contend it required Congress to formally approve all uses of U.S. military force abroad, save, as James Madison said at the Convention, in situations needed to "repel sudden attack." Others maintain the "declare war" clause provides more leeway, allowing the president to use force abroad as long as the force does not rise to the level of "war," whatever that means. Yet others argue that the framers meant simply to give Congress the authority to signal under international law a state of war; the real work in controlling presidential initiation of force, under this view, was Congress' control over appropriations and the size of the standing army. There are many more theories about the original understanding. Even if we could definitively resolve this debate, which we can't, it is unclear why original intent—which in practice rarely determines contemporary constitutional meaning—should control outcomes in the context of presidential war powers, a context that as much as any is marked by radically changed circumstances.
Compounding the problem of indeterminate constitutional language is the fact that the courts have never resolved the question about the scope of the president's power to use military force abroad without congressional authorization. Almost all litigation seeking to resolve whether a war was properly launched has been dismissed as a "political question" or because the plaintiff lacked standing. As a result, the constitutional issue has been worked out almost exclusively by practice between the political branches and not by the courts.
Eric Posner suggests that Obama is doing nothing that previous presidents have not done before him (the imperial presdiency may now be institutionalized), and suggests further that Congress should play a limited role :
President Obama is following a long line of precedents in which the executive lanched a foreign war without congressional authorization. The president disavowed these precedents during his campaign; he may or may not attempt to distinguish his campaign statement by invoking the UN security council resolution authorizing the attack, as Truman did for Korea. But this legal wrangling is all superstructure. Congress is disabled in numerous ways from making practical contributions to a war effort. It cannot prevent the president from starting a war, and it is nearly impossible to halt an ongoing war. Wars, then, simply become an opportunity for members of Congress to stake their reputations as hawks or doves for the sake of future elections.
. . . Congress could not play a role. Lacking a leader who could commit it to a course of action, Congress could not make promises. Lacking a single mouthpiece, it could not be consulted. Foreign countries naturally turned to the president. Nor is it realistic for Congress to formally ratify the president’s decision if formal ratification involves the possibility of rejection. Then the next time that the United States is involved in a foreign policy crisis, other countries won’t know who to speak to, and who to believe.
As we've argued in class, the stucture of the executive seems to put it in a better position than Congress to make and implement decisions about foreign affairs. Its a matter of design.
