One of the critical differences between the liberal and conservative foreign policy traditions is their willingness to accept that America is capable of evil. For conservatives -- as I write in my new book, The Good Fight: Why Liberals--and Only Liberals--Can Win the War on Terror and Make America Great Again -- the idea has traditionally been anathema. From the beginning of the modern conservative movement in the 1950s, conservatives have worried that if Americans recognized their moral fallibility, they would lack the self-confidence to fight their enemies. That's why conservatives largely refused to admit that Joseph McCarthy was violating civil liberties. It's why when Bush gets called on Guantanamo, he basically says, that's ridiculous, we're America, we can't violate human rights.

This horrible story from Haditha powerfully underscores the liberal vision, which is this. We are not angels: without sufficient moral and legal restrictions, and under conditions of extreme stress, Americans can be as barbaric as anyone. What's makes us an exceptional nation with the capacity to lead and inspire the world is our very recognition of that fact. We are capable of Hadithas and My Lais, so is everyone. But few societies are capable of acknowledging what happened, bringing the killers to justice, and instituting changes that make it less likely to happen again. That's how we show we are different from the jihadists. We don't just assert it. We prove it. That's the liberal version of American exceptionalism, and it's what we need right now in response to this horror.