The fact is, unless the nation is invaded or its very survival is imminently threatened, going to war is always a choice. So what is the point of trying to make this elusive distinction anyway? For many, including Obama, the present purpose is to distinguish Afghanistan from Iraq, Obama’s “good” war from George W. Bush’s “bad” war. But it won’t work. As Haass correctly argues, right or wrong, they were both wars of choice.

But there is a deeper reason, as well, for Obama to claim necessity in Afghanistan. It is part of what increasingly seems to be a striving for moral purity in international affairs by this administration. Obama and his top advisers apologize for America’s past sins, implicitly suggesting they will commit no new ones. And that goes for fighting wars. No one can blame you for fighting a war if it is a war of necessity, or so they may believe. All the inevitable ancillary casualties of war — from civilian deaths to the occasional misbehavior of the troops to the errors of commanders — are more easily forgiven if one has no choice. The claim of necessity wipes away the moral ambiguities inherent in the exercise of power. And it prevents scrutiny of one’s own motives, which in nations, as in individuals, are rarely pure.

This hoped-for escape from moral burdens is, however, an illusion. Just because America declares something necessary doesn’t mean that the rest of the world, and especially its victims, will believe it is just. The claim of necessity will not absolve the United States, and Obama, from responsibility for its actions.