I've been trying to picture what the world might look like the day after a U.S. nuclear strike on Iran, but I'm essentially drawing a blank. There simply isn't a precedent for the world's dominant superpower turning into a rogue state – much less a rogue state willing to wage nuclear war against potential, even hypothetical, security threats. At that point, we'd truly be through the looking glass.
One can assume (or at least hope) that first use of nuclear weapons would turn America into an international pariah, at least in the eyes of global public opinion. It would certainly mark the definitive end of the system of collective security – and the laws and institutions supporting that system – established in the wake of World War II. The UN Security Council would be rendered as pointless as the old League of Nations. The Nuremberg Principles would be as moot as the Geneva Conventions. (To the neocons, of course, these are all pluses.)
Nuclear first use would also shatter (or at least, radically transform) the political alliances that defined America's leadership role in the old postwar order. To the extent any of these relationships survived, they'd be placed on roughly the same basis as the current U.S. protectorate over Saudi Arabia – or, even worse, brought down to the level of the old Warsaw Pact. They would be coalitions of the weak, the vulnerable and the easily intimidated.
In other words, the current hegemony of American influence and ideas (backed by overwhelming military force) would be replaced by an overt dictatorship based – more or less explicitly – on fear of nuclear annihilation. U.S. foreign policy would become nothing more than a variation on the ancient Roman warning: For every one of our dead; 100 of yours. Never again would American rulers (or their foreign counterparts) be able to hide behind the comfortable fiction that the United States is just primus inter pares – first among equals. A country that nukes other countries merely on the suspicion that they may pose a future security threat isn't the equal of anybody. America would stand completely alone: hated by many, feared by all, admired only by the world's other tyrants. To call that a watershed event seems a ridiculous understatement.
It's time to wake up, America.