Is the military option to strike Iran (legally) on the table?
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As major hurdles appear to remain in coming to an agreement with Iran to bar its acquisition of nuclear weapons, discussion has reignited about the "military option" should the diplomats fail to reach a settlement. President Obama has long declared that the military option was on the table as a last resort, and as recently as last April, Defense Secretary Ashton Carter seemed to reaffirm that position. He insisted that the U.S. has "the capability to shut down, set back and destroy the Iranian nuclear program," adding that he believed that "the Iranians know that and understand that."

Could the military option actually "destroy" the Iranian program? My guess is that it could, but a campaign to do so would likely be long, hard, costly and carry unintended consequences. However, the even tougher question is this: What would be the legal basis for it?

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Some time ago, I looked at that issue and concluded then that there was an inadequate factual basis for international law to justify the use military force. Despite the passage of time, I still believe that to be the case. Why? Let's look at the law. Since the adoption of the U.N. charter, there are basically only two real justifications for a nation to use force against another country.

The first is when the United Nations' Security Council authorizes military force, but a veto by one of the permanent members — such as China or Russia — would frustrate that route. Given that it is quite possible that either would use its veto to stall "snapback" sanctions should an agreement with Iran go awry, it is almost unthinkable that a veto would not also be used to block a military effort to halt the Iranian nuclear program.

The other option is to act in self-defense. Article 51 of the U.N. charter says that "Nothing in the present charter shall impair the inherent right of individual or collective self-defense if an armed attack occurs against a member of the United Nations." While this provision has been widely interpreted to permit "anticipatory self-defense," it typically would require evidence of an "imminent" threat.

History may help us here. In 1981, Israel attacked and destroyed a nuclear reactor in Saddam Hussein's Iraq, basically employing the "anticipatory self-defense" rationale. However, the U.N. Security Council — to include even the U.S. — unanimously approved a resolution condemning the raid.

Ambassador Walter Stoessel, then acting secretary of State, told Congress that the U.S. had condemned the attack because it believed that the "the Israelis had not exhausted all the diplomatic options available for alleviating their concerns." He added that the "unprecedented character of the Israeli air attack could not but seriously add to the already tense situation in the area and seriously complicate [the U.S.'s] effort to resolve the various problems in the area through peaceful means." Would that not also be the case today?

It is true that, given the terrible threat of weapons of mass destruction, and the technology-enabled compressed timeline in which actors might use them, the U.S. in recent years has taken a more "flexible" approach to what constitutes "imminence." That has caused some experts to conclude that a strike might be legal, but I simply do not believe that the available facts — as we know them today — would constitute the necessary "imminence" to justify a strike.

Even fairly aggressive estimates of Iran's "breakout time" for acquiring a weapon are in the range of at least several months. In addition, the weapon would need to be mated with a delivery capability, which is possible, but would take some period of time.

Possession of a weapon and delivery capability is, however, only one part of the equation, as imminence requires an intent to use it in the near term. Who would say, for example, that a strike against a nation (like the U.S.) with an inventory of nuclear weapons would be lawful, simply based on the possession of the weapons and the means of delivering them?

It is important to keep in mind that the mere possession of the devices by the U.S. (and other nations) is not intrinsically illegal under international law. In fact, the International Court of Justice has even concluded that it could not say that their use would be illegal under extreme circumstances. Yes, the acquisition of a weapon by Iran would violate the Nuclear Nonproliferation Treaty and U.N. resolutions, but these instruments do not themselves authorize the use of force for breaches. Likewise, we need to recognize that even if Congress were to "declare war" on Iran, that kind of domestic legal basis does not necessarily translate into authority under international law.

None of this is to minimize the very real concerns about the Iranian nuclear program, nor does it foreclose the propriety of the use of force should the facts change (or, for that matter, if the government currently has classified information showing the threat to more imminent than we may suppose). Of course, we also need to appreciate that the seeming availability of the military option can be a vitally important negotiation tool.

Still, if the U.S. (or Israel) should actually use force based on the facts publicly available now, doing so would likely have a profoundly unsettling effect on international norms that since the establishment of the United Nations have helped prevent, if not every armed conflict, major interstate war. When we are talking about nuclear-armed nations that is a norm of no small importance, and one worth doing everything possible to preserve.

Dunlap is a retired Air Force major general and currently professor of the practice of law at Duke University School of Law, where he is also executive director of the Center on Law, Ethics and National Security.