In practice, there is no obvious multilateral forum through which Obama can act to tackle the crisis in Libya. Until this week, Washington was hoping the Libyan rebels might win by themselves. But now Gaddafi’s troops are advancing, he has more weapons, more mercenaries, and apparently a lot of cash. A Gaddafi victory, culminating in a slaughter, would be a disaster for Libya, and would set a disastrous precedent in the Arab world and beyond. Other tyrants will conclude that Mubarak and Ben Ali were wrong, that the way to stay in power is to be even more brutal and kill even more people. Here is the irony: a Gaddafi victory would also be disastrous for Obama. Libyan rebels will accuse him of colluding with Gaddafi. Domestic opponents will accuse him of weakness.

In the next few days, Obama will have to decide whether to enforce a no-fly zone or – and this may be the more likely solution – to offer food, weapons and political support to the rebels, probably in conjunction not with the UN but with an ad hoc “coalition of the willing”, as George W. Bush would have put it. There are heavy costs to saying yes, and heavy costs to saying no. Either way, Obama is about to learn a lesson: because of America’s size and military power, the American president does not have the option to remain neutral indefinitely, to let others lead or to offer mere moral encouragement – even though those are the policies this president would prefer.