President Clinton lied in his 1998 apology to survivors of the Rwandan massacre when he suggested that he and his staff hadn't known genocide was taking place. Documents obtained subsequently under the Freedom of Information Act in 2004 by activist groups showed that the Clinton administration referred to the slaughter as "genocide" in its internal discussions but refused to say so publicly because Clinton had decided against intervention.

"Genocide can occur anywhere. It is not an African phenomenon," he said in 1998 as part of his apology. "We must have global vigilance. And never again must we be shy in the face of the evidence." Thus, Clinton nicely articulated a moral principle whose moral authority he excluded himself from.

Nonetheless, this principle has saturated much of the recent discussion about Darfur. Indeed, as historian and columnist Niall Ferguson noted, Obama called for an increased military commitment in Sudan, including possibly sending NATO, in order to prevent genocide just two years ago.

There's been so much talk about how conservative foreign policy's moral credibility has been demolished under President Bush. Maybe. But what of liberal credibility? In the 1990s, amid the debates about Haiti, Somalia, Yugoslavia and Rwanda, the broad outline of the debate had conservatives advocating a narrower definition of the national interest while liberals argued - and I often agreed with them - for a more expansive one that included a heavy dose of moralism. Finally, liberals seemed to have shaken off the Vietnam syndrome and embraced an overly optimistic but benign foreign policy of nation-building and do-goodery.

Conservatives are at least still arguing about the national interest - but they're also the ones touting the moral imperative of preventing genocide and even the need for nation-building. Where is the principle in the hash of liberal foreign policy today? How does liberalism recover? If you can justify causing genocide in order to end a nation-building exercise that - unlike similar efforts elsewhere - is fundamentally linked to our national interest, then how can you ever return to arguing that we should get into the nation-building and genocide-stopping business when it's explicitly not in our interest?