University of Notre Dame
Kroc Institutde for International Peace Studies

Mary Ellen O’Connell

Mary Ellen O’Connell is the Robert and Marion Short Professor of Law and Research Professor of International Dispute Resolution at the University of Notre Dame’s Kroc Institute for International Peace Studies.

Opponents of the Iran nuclear deal have attacked the treaty in part by claiming there are better alternatives. They argue they can get a better treaty by re-imposing tough sanctions, and some even claim that military force could be used instead of a treaty to eliminate the nuclear program. Treasury Secretary Jacob Lew made it clear in a New York Times op-ed in August that new sanctions are a non-starter. Other countries are unlikely to join the U.S. in re-imposing sanctions. More sanctions will hurt the U.S. with little chance of persuading Iran.

That leaves military force. Indiana Senator Joe Donnelly explained his vote for the treaty by saying he wanted to try diplomacy before going to war: “With or without this deal, the day may come when we are left with no alternative but to take military action to prevent Iran from crossing the nuclear threshold.”


The truth is that the United States has no legal right to go to war to stop Iran from building a nuclear weapon. No state does. The use of military force is prohibited without UN Security Council authorization except in cases of self-defense—“if an armed attack occurs.” The Council has the power to authorize force in response to threats to the peace, as it did in 2011 in Libya but declined to do in 2003 for Iraq. In the case of Iran’s nuclear program, even the Security Council is limited in considering the use of force by the principle of necessity. Under this principle, derived from Just War Doctrine, the use of military force is prohibited where there is little or no chance of success and where force is not a last resort.

More sanctions will hurt the U.S. with little chance of persuading Iran.

These legal points on war are critical because the Iran deal itself is a treaty under international law. Every nonproliferation obligation required of Iran is derived from the binding nature of international law. To treat the rules as required for Iran but only optional for others is to press arguments that undermine the very system of international order states are attempting to uphold in the Iran agreement.


For the U.S. and every sovereign state, international law matters significantly in questions of war and peace. Where the U.S. has failed to take international law seriously it has paid a steep price. In 2003, the United States, together with the United Kingdom and Australia, invaded Iraq. U.S. government lawyers tried to assert in a letter to the Security Council that the war was permissible under international law. The letter invoked Security Council resolutions authorizing the use of force to liberate Kuwait in 1990-91, which included prohibitions on Iraq from obtaining weapons of mass destruction.

The U.K. government’s top two international lawyers counseled that the 1991 Kuwait resolutions could not be used in 2003 as the legal basis for invading. Prime Minister Tony Blair ignored that advice. Most of the world, however, agreed with the U.K.’s lawyers, interpreting the law as requiring new authorization. When the invasion went forward only three states sent troops.

Through international law and cooperation, nuclear arms proliferation can be stopped.

In 1991, by contrast, virtually all states supported military action against Iraq in defense of Kuwait. Kuwait was liberated in 100 hours of combat, and it remains a free and independent state. The advantages of complying with international law are obvious. The use of UN sanctions and weapons inspection in the 1990s was successful in preventing Saddam Hussein from getting weapons of mass destruction. That case and others demonstrate that through international law and cooperation, nuclear arms proliferation can be stopped.

The Agreed Framework reached in 1994 to end North Korea’s program showed promise initially, but it failed partly because the United States did not fulfill its promises to lift sanctions and normalize political relations. We will never know if North Korea could have been stopped. We do know North Korea was not legally obligated to comply with the agreement in the face of the breach from our side.


In the case of Iran, nobody should expect the Security Council to approve military force. The principle of necessity requires a showing of a high likelihood of military success. That case cannot be made. Many experts doubt that bombing would be able to end Iran’s program; it is more likely to accelerate it. Bombing will also inflict a terrible cost on the Iranian people. The belief in most of the world is that diplomacy works and is the best route to arms control.

If the United States attacks unilaterally or with one or two allies, the U.S. will be the law violator, not Iran. Even discussing the possibility of military action is foolhardy because it weakens our demands that Iran abide by its treaty obligations and Security Council mandates. Loose talk of illegal attack undermines U.S. criticisms of Russia in Ukraine and China in the South China Sea and cyberspace. In the history of successful arms control diplomacy, legal agreements, and international cooperation do work. Military force does not.