But what if moral revulsion and geopolitical realism holds? What if the only nukes that are used turn out to be the ones the U.S. uses on Iran, at least in the immediate aftermath? The consequences for the U.S. could well be severe and lasting. Much of the world is already troubled if not disgusted with recent U.S. international behavior. (See for example the dKos diary by NBBooks that outlines several growing alliances that don’t include the U.S.) The use of any nuke could unleash a tide of sentiment and action that could devastate the U.S. politically and economically. Nuclear weapons still have potent symbolic as well as physical power.

The U.S. could become an outcast giant overnight, drummed out of the international community, treated with contempt. Real penalties could be exacted through the UN and other international bodies. Now that other nations are economically strong and America makes little of what the world needs, there is less incentive for allowing this violation—this most violent single act since World War II-- to be forgiven and forgotten. As our debts are called in, America may find that its chief exports—Hollywood, weapons and garbage—are no longer sufficient to balance its offenses.

Even if nations are cowed into silence by the U.S. willingness to use its greatest remaining source of world power—its nuclear arsenal—terrorism against the U.S. would undoubtedly increase, but the rest of the world will turn a blind eye.

To break the nuclear peace is potentially the most consequential single act possible. For it is only the remarkable shared forbearance on the use of nuclear weapons, a forbearance unique in human history, that has allowed civilization to continue. No matter how I look at it, it’s hard to see this any other way: the day that America uses a nuclear weapon against Iran will be the darkest day in American history.