[Niall Ferguson is a Harvard professor and Newsweek columnist.]

“If it were done when ’tis done, then ’twere well it were done quickly.” Macbeth’s famous line before he kills Duncan came to mind last week, when President Obama belatedly changed his mind about military intervention in Libya. Like Obama, Macbeth fervently hopes that “this blow might be the be-all and the end-all”:

But in these cases … we but teach
Bloody instructions, which, being taught, return
To plague the inventor: this even-handed justice
Commends the ingredients of our poison’d chalice
To our own lips.

The president has been more Hamlet than Macbeth since the beginning of the revolutionary crisis that has swept the desert lands of North Africa and the Middle East. To act or not to act? That has been the question. The results of his indecision have been unhappy. Hosni Mubarak, for so long an American ally, has been overthrown in Egypt. Muammar Gaddafi, the erstwhile sponsor of terrorism so foolishly rehabilitated by the West just four years ago, has—until now—lived to fight another day in Libya. Meanwhile, in Bahrain, another insurrection is being quelled with the help of Saudi Arabia—an American ally even more important than Libya....