During his March 4 AIPAC speech, President Barack Obama came closer than ever before to declaring that should sanctions fail, he was prepared to use military force to stop Iran from acquiring nuclear weapons. He explicitly rejected the idea that the U.S. should base its future approach on deterring a nuclear Iran, stressing that his policy was preventing a nuclear Iran: “Iran’s leaders should know that I do not have a policy of containment; I have a policy to prevent Iran from obtaining a nuclear weapon.”

Obama then listed the efforts his administration had undertaken, concluding the list with, "and, yes, a military effort to be prepared for any contingency." He repeated, "I will take not options off the table,” adding, "and I mean what I say." But there was no explicit guarantee that the U.S. would attack if Tehran reached the point of assembling a weapon. Defense Secretary Leon Panetta clarified the administration´s policy two days later: “Military action is the last alternative if all else fails, but make no mistake: When all else fails, we will act.”

Did this mean that the Obama administration was indeed prepared to launch a preventive strike at Iran´s nuclear facilities in the future? If that was the case, this would represent a sharp break from the position of many of the critics of the 2003 Iraq war who rejected the legal right of the U.S. undertake such attacks.

These critics were mostly found in American academia and a number of leading law schools, Obama´s milieu before he entered politics. They included highly respected scholars like Harold Koh, the dean of Yale Law School, who would become the legal adviser of the State Department under Obama.

In the shadow of 9/11, it was the 2002 Bush Doctrine that asserted the U.S. right to engage in preventive attacks most forcefully, when it spoke about "taking the battle to the enemy ... to confront the worst threats before they emerge.” In contrast, the famous Article 51 of the U.N. Charter asserts an "inherent right of self-defense if armed attack occurs against a member of the United Nations.”

International legal scholars, for the most part, recognized a right of pre-emption as far back as the 19th century, when Secretary of State Daniel Webster detailed the pre-conditions for pre-emptive strikes after the British attacked an American steamer, the Caroline, along the U.S.-Canadian border. Israel´s attack in the 1967 Six-Day War demonstrated again the legitimacy of pre-emption when it appeared that war was about to break out.

But Bush took this a step further, from pre-emption to prevention, by saying that America was not going to wait to the last minute before acting, but rather would neutralize threats well before they became imminent.

Within two years, Bush´s ideas were forcefully rejected, especially in liberal circles, as U.S. forces became bogged down in the Iraqi insurgency. The New York Times published an editorial in September 2004 entitled: "Preventive war: a failed doctrine." Along with Harold Koh, Professor Michael Doyle of Columbia University convened a seminar in 2008 under the prestigious Carnegie Council, which he opened by saying, "Talking about preventive self-defense today, in the wake of the Iraq fiasco, is something like interviewing the passengers of the lifeboats of the Titanic about their views on ocean travel." It seemed that the U.S. was not again going to take military action so quickly against a rogue state developing nuclear weapons, as it did in Iraq.

There were two main legal arguments repeatedly voiced against preventive military actions by the U.S. First, the threat it was seeking to neutralize was not imminent, but was still being formed. Alan Dershowitz explained in his 2006 book, “Pre-emption,” that there was a consensus that such preventive attacks against non-imminent threats were illegal according to international law.

But should pre-emption and prevention be treated so differently considering that the real difference between them is how far away the threat they are addressing appears on a timeline?

Today, moreover, there is a growing problem of waiting to the last minute for an imminent threat. In the conventional battlefield, imminent threats are visible. There are classic signs intelligence services can pick up weeks before a war, like reserve mobilization, or the movement of forces and ammunition stocks from their regular bases to forward positions.

But in the push-button era of missiles, it is much harder to know that an enemy is preparing an imminent attack, in which case a pre- emptive strike might be considered. Moreover, the risks of waiting until those preparations become evident are much too great with nuclear weapons. For that reason, there have been efforts underway to update international law.

At this time, Obama is not prepared to take preventive action against Iran precisely because he believes he has plenty of time. He told The Atlantic´s Jeffrey Goldberg in a recent interview: "Iran does not yet have a nuclear weapon and is not yet in a position to obtain a nuclear weapon without us having a pretty long lead time in which we will know that they are making that attempt.”

But is Obama´s sense of confidence about the ability of intelligence services to warn him in time warranted? Two years ago, then Defense Secretary Robert M. Gates was discussing the Iranian nuclear program and asked: “If their policy is to go to the threshold but not assemble a nuclear weapon, how do you tell that they have not assembled? I don’t actually know how you would verify that.”

Gates fully understood the limits of intelligence; in the 1990s he headed the CIA. The import of what Gates was saying is that by the time the U.S. knows whether Iran has crossed the nuclear threshold, it might be too late to take any action.

The second legal argument against the doctrine of preventive operations from the Bush era is that they were unilateral, without the backing of the U.N. Security Council. The Obama administration´s official National Security Strategy allows for American unilateralism. But in reality the situation is more complicated, as in the case of Libya, in which the U.S. still relied on a U.N. mandate with NATO support.

Legal scholars who now assert the legitimacy of preventive strikes in many cases insist that the evidence against rogue states be first presented to the U.N. Security Council, despite the well-known delays that the U.N. machinery has demonstrated in repeated crises. It should be stated that past U.S. governments have used force without U.N. authorization: from Kennedy´s naval quarantine around Cuba to Reagan´s air attack on Libya to Clinton´s missile strikes on an alleged chemical weapons factory in Sudan. None of these attacks involved an imminent threat of attack on the U.S.

It is not so clear that the Obama administration is so willing to shed the requirement of U.N. authorization. During testimony he gave before the Senate Armed Services Committee on March 7, Defense Secretary Panetta stated that in the case of Syria, before the U.S. could become militarily involved, "our goal would be to seek international permission." Certainly, the Syrian people who are under siege would prefer not to have their rescue dependent on the goodwill of Russia and China in the Security Council.

By the same reasoning, would effective action against Iran be made dependent on an international consensus at the U.N. that does not even exist on sanctions? Undoubtedly, the Obama administration declarations have shed much of the reluctance after the Iraq war to consider preventive military action when confronting a challenge on the scale of the Iranian nuclear program.

But the rhetorical shift is not enough. In practice, it appears that even if it becomes clear that sanctions have had no impact on Iranian decision-making with respect to nuclear weapons, it will still take a very long time until a decision to use U.S. force to halt Iran will be made.