At the National Defense University Monday, President Obama made a strong case for intervening in Libya, arguing that morality and national security demanded it. For much of the speech, he seemed to blur the distinction between the two, obscuring his views on whether and how ethics and self-interest interacted in his decision to use force. But then he said this:

We knew that if we waited one more day, Benghazi — a city nearly the size of Charlotte — could suffer a massacre that would have reverberated across the region and stained the conscience of the world….

To brush aside America’s responsibility as a leader and — more profoundly — our responsibilities to our fellow human beings under such circumstances would have been a betrayal of who we are....And as president, I refused to wait for the images of slaughter and mass graves before taking action.

Moreover, America has an important strategic interest in preventing Gaddafi from overrunning those who oppose him. A massacre would have driven thousands of additional refugees across Libya’s borders, putting enormous strains on the peaceful — yet fragile — transitions in Egypt and Tunisia. The democratic impulses that are dawning across the region would be eclipsed by the darkest form of dictatorship, as repressive leaders concluded that violence is the best strategy to cling to power. The writ of the U.N. Security Council would have been shown to be little more than empty words, crippling its future credibility to uphold global peace and security.

Some excellent points. And, making good on this logic, he admitted that America’s humanitarian and strategic objectives in Libya now can’t be fully realized while Gaddafi maintains power. So what’s the strategy to get rid of him?

We will deny the regime arms, cut off its supply of cash, assist the opposition, and work with other nations to hasten the day when Gaddafi leaves power. It may not happen overnight, as a badly weakened Gaddafi tries desperately to hang on to power. But it should be clear to those around Gaddafi, and to every Libyan, that history is not on his side. With the time and space that we have provided for the Libyan people, they will be able to determine their own destiny, and that is how it should be.

What does “assist the opposition” mean? Money? Training? Arms? A robust embargo against Gaddafi and his henchmen? Kind words? So far, America hasn’t seemed enthusiastic about much beyond the last two.

The “time and space” that the coalition is providing the Libyan rebels are the results of a large coalition effort; will the varied nations involved be able to sustain it as Gaddafi “tries desperately to hang on to power”? Or will America have to decide whether it can shoulder more of the cost when others lose interest?

The plan bets on the relatively quick success of the Libyan opposition and the coalition’s ability to promote rebel efforts from afar. It could work. But if one or both of those are inadequate, we could we be patrolling Libya’s skies for a long time yet. The president didn’t fully explain why he doesn’t think this will happen.