"Knowing what we know now." With those five words, virtually the entire 2016 Republican presidential field erased over 12 years of GOP talking points steadfastly supporting President Bush's 2003 invasion of Iraq. But that phrase is much less important in what it says about the past (after all, as many have noted, we had plenty of evidence at the time that the U.S. invasion wouldn't unearth WMD but instead only detonate sectarian chaos in Iraq) than what it means for the future. That's because many of the same voices now belatedly acknowledging the catastrophe of America's preventive war against Iraq are calling for another preventive war of choice against Iran.
That's right. To choose to start a war to prevent a possible threat in the future--as opposed to preempting an imminent enemy attack--policymakers have to be right. Right about the nature and the likelihood of the threat. Right about the legal basis in U.S. and international law. And just as important, right about the consequences of launching a war they could have chosen not to fight.
And when it comes to the Iranian nuclear program, the United States is rapidly coming to that same fork in the road. If the Obama administration fails to secure a deal with Tehran, or if an agreement is sabotaged by opponents in Congress, then Americans will have to choose. Which is worse: living in a world with a nuclear Iran or fighting a regional (and possibly global) war to prevent it?
For the advocates of strikes against Tehran nuclear facilities, war with Iran would be little more unpleasant than an ill-timed fart. All it would take, grandstanding Arkansas Republican Senator Tom Cotton declared, is "several days of air and naval bombing." But that it not the consensus of national security experts here and in Israel. As I summed it up in March, the cost would be staggering for combatting the subsequent Iranian retaliation in the Persian Gulf, across the region, in Europe and even here:

[The cost] could be as much as $2 trillion over a decade. Thousands of U.S servicemen and women, as well as American civilians, could be the casualties of a conflict that might well spread beyond the region. To ensure that Iran can never develop nuclear weapons, that's the possible price tag in blood and treasure for an American invasion and occupation of Iran that would require "a commitment of resources and personnel greater than what the U.S. has expended over the past 10 years in the Iraq and Afghanistan wars combined."

Remember, the last time we traveled this path, we were informed that Iraqi oil revenues would ultimately pay for the cost of the conflict and estimates that the American occupation would require "several hundreds of thousands" of U.S. troops were dismissed as "wildly off the mark."
And to be sure, President Bush's invasion of Iraq was an exercise in preventive war. In his October 7, 2002, address in Cincinnati, Bush warned, "Facing clear evidence of peril, we cannot wait for the final proof--the smoking gun--that could come in the form of a mushroom cloud." That echoed the talking point National Security Adviser Condoleezza Rice mouthed a month earlier, when she fretted, "We don't want the smoking gun to be a mushroom cloud." Addressing the Veterans of Foreign Wwars nearly six months before Colin Powell would make his infamous presentation to the United Nations, Vice President Dick Cheney was unequivocal about the future threat from Saddam Hussein:

Simply stated, there is no doubt that Saddam Hussein now has weapons of mass destruction. There is no doubt he is amassing them to use against our friends, against our allies, and against us. And there is no doubt that his aggressive regional ambitions will lead him into future confrontations with his neighbors -- confrontations that will involve both the weapons he has today, and the ones he will continue to develop with his oil wealth.

For his part, John McCain was on board 100 percent. He didn't just agree that the Iraq war would be a short one and that Americans would be "greeted as liberators." Three months after the invasion in June 2003, McCain announced:

I remain confident that we will find weapons of mass destruction in Iraq.

But it didn't work out that way. Bush, Cheney, Rice and McCain (among others) were, as Iraq Survey Group Charles Duelfer testified in October 2004, "almost all wrong." And not just about WMD, but about Saddam's links to Al Qaeda and pretty much everything else. To have any legitimacy in international law and in the court of world opinion, the justifications for preventive war must be true and the "gathering threats" real ones. The Bush administration failed on every criterion.
To put it another way, if any idea should have been thoroughly discredited by the blood and treasure lost in ousting Saddam Hussein and the subsequent carnage in Iraq, it is the very notion of preventive war itself.
Whether or not preventive war constitutes legitimate self-defense under international law, history is replete with examples that did not end well for the aggressors. (For Americans, the Japanese attack on Pearl Harbor should leap to mind.) While "pre-emption" is "meant to grab the tactical advantages of striking first against what is seen as a truly imminent threat, when an adversary's attack is close at hand," the Oxford Bibliographies explained that:

The strategic logic of preventive war is rooted in the desire to halt the erosion of relative power to a rising adversary and the future dangers this power shift might present. Leaders calculate that a war fought in the near term will be less costly than a war fought at a later date, after the potential adversary has had an opportunity to increase its military capabilities. Under preventive war conditions, there is no certainty that this future war will actually be fought; preventive war is launched to avoid the mere possibility of a higher-cost future war or the potential for the target state to use its rising power in a coercive way.

Nevertheless, if the Republicans (and some Democrats) now attacking President Obama have their way, the United States will be on a course for a new preventive war, this time against Iran. And if the negotiations with Tehran over its nuclear program falter, President Obama will have to face the same challenge he issued two years ago. As Obama cautioned in March 2012, "This is not a game," he said. "And there's nothing casual about it."

"If some of these folks think that it's time to launch a war, they should say so. And they should explain to the American people exactly why they would do that and what the consequences would be."

Or as General David Petraeus put it during the run-up to the invasion of Iraq, "Tell me how this ends."
Sadly, we still don't know, as the final chapter in America's cataclysm in Iraq has yet to be written. But we do know this. In place of the secular Sunni dictator Saddam Hussein, the U.S. enabled the creation of a Shiite-dominated state loyal to Tehran in its place, the central government of which soon alienated the very Sunni tribes that largely ejected Al Qaeda during the "Surge." As a result, Iraq has devolved into an ongoing sectarian bloodbath, as the toxic blend of Saddam loyalists and former Al Qaeda fighters found support on either side of the Syrian border and put both the Kurds and the government in Baghdad at risk. Oh, and one other thing. Forty-four hundred Americans troops were killed, over 30,000 wounded and perhaps hundreds of thousands Iraqi civilians left dead.
After a four day reversal that left him with a painful whiplash, Jeb Bush on Friday finally reached this conclusion:

"Knowing what we now know, what would you have done? I would have not engaged. I would not have gone into Iraq."

But soon, all Americans may have to answer a different question. Knowing what we know now, what about Iran?