The UK government is apparently working on a contingency draft Security Council resolution authorizing a no-fly zone for Libya. That kind of contingency planning is useful as far as it goes. For one, it helps find what the potential “red lines” are for countries like Russia that have so far expressed reservations about the idea.
But it is worth noting that without a Security Council resolution, it really does not make sense for the United States to lead a no-fly zone over Libya.
Right now, Qaddafi is the most isolated leader in the world. His actions have lead to a series of diplomatic measures that are unprecedented for the kind of international support received. The Security Council resolution, which passed unanimously, included for the first time ever all five veto-wielding members of the Council approving ICC action. The General Assembly, by consensus, agreed to boot Libya from the Human Rights Council — that means literally every country in the world ganged up against Qaddafi. So great is this moment of international unity that even Israel and Palestine have joined forces against him.
A fractured Security Council would erode that international consensus, and leave the United States to enforce the no-fly zone by itself or with a relatively small coalition of the willing (not likely to include, NATO). The United States would constantly have to defend the decision to work outside the council (and outside NATO) while simultaneously looking to the council for support on a host of other issues, from non-proliferation to peacekeeping to North Korea.
It would probably also divide the Arab world, and in so doing undermine what are arguably more important American interests. To name a few: ensuring a steady transition to democracy in Egypt; progress on Israel and Palestine; and working to ensure that this wave of revolution spreading across the Arab world brings about governments that, at the very least, are not hostile to the United States.
So all those interests, and others, have to balanced against the question of whether leading a unilateral military assault is more vital to the United States than these other issues? (Keep in mind: Libya is not a major exporter of oil to the United States; Libya is not a strategically vital country in terms of its geography; it contains no weapons of mass destruction and does not pose a threat to the United States homeland or our allies in the region.)
With a Security Council resolution, these kinds of choices between interests are simply not as stark. You don’t have to choose between angering half the world. Military intervention would have both legal and political legitimacy and enjoy broad (if not unanimous) backing by the international community. That is because if such a measure were to pass the Security Council, it would probably have to have the backing of relevant regional organizations like the Arab League and African Union. (Countries like Russia and China that are generally more non-interventionist tend to drop their objections if regional organizations support the measure.) Out of necessity, the United States would probably still play a lead role, but it would not be the face of the intervention.
If you think military intervention is the way to go (and for the record, I am personally not yet convinced) a Security Council resolution is not something that is nice to have. It is a must. Otherwise, it is exceedingly difficult to see how such a mission would at all be in the service of American interests.