JURIST Contributing Editor Mary Ellen O'Connell of Notre Dame Law School says that the United States today has no legal basis to use significant armed force against Iran, and that another unlawful war in the wake of the Iraq debacle would be very damaging to American interests and image abroad...
International law provides only two bases for the lawful use of force: self-defense and Security Council authorization. The Security Council refused to authorize America's tough economic sanctions package against Iran, so there is little likelihood it would authorize a military strike. 
As for self-defense, in an NPR interview on January 31, Under Secretary of State Nicholas Burns invoked United Nations Charter Article 51 with respect to our relations with Iran (although he also said the U.S. is not planning a war).  Article 51 provides an exception to the Charter's general prohibition on the use of force. States have the right to use force in individual and collective self-defense if an armed attack occurs. Secretary Burns seemed to suggest that such a right exists vis-Ã -vis Iran. In fact of all Iran's many infractions, none qualify as a trigger for the right of self-defense, meaning that there is no right to use major military force on Iran's territory.
The International Court of Justice (ICJ) clarified in the 1986 Nicaragua Case that it is not just any attack that triggers the right of self-defense: the attack must be significant. The Court said "the concept of armed attack includes â¦ acts by armed bands where such acts occur on a significant scale."  The Court said that the supply of weapons did not amount to an armed attack nor could mere frontier incidents give rise to the right to use force in self-defense. 
Moreover, to be acting in lawful self-defense, any response to a significant attack must be defensive in nature. States may not act out of motives of revenge, future deterrence or punishment. The victim of an initial attack may respond in self-defense if there are on-going attacks or, at the least, if it has clear and convincing evidence that more attacks are imminent.
The Court also said there is no right of "collective counter-measures." In other words, while a victim of a significant armed attack may request the help of other states in collective self-defense, there is no collective right of response to lesser wrongs.  The Court added that a victim state must formally request other states to join it in collective self-defense. [ 7] The Court was concerned to limit third state involvement in conflicts and interpreted the Charter provisions on collective self-defense narrowly. 
This is all common sense. The rules on the use of force in international law are intended to restrict armed conflict as much as possible. International law is a peace regime, not a war regime. The right of self-defense is an exception for emergency situations only. Indeed, the right to use force in self-defense ends when the Security Council takes effective action.
The Bush Administration is complaining about several actions by Iran at the current time. Iran is allegedly supplying weapons and training to fighters inside Iraq.  It may be doing the same regarding Hezbollah fighters in Lebanon. It has sponsored acts of terrorism, such as bombing a Jewish community center in Argentina.  Iran is defying the Security Council by refusing to end its program for enriching uranium.  These are all serious international wrongs; none of them, however, may be righted by the significant use of American force on the territory of Iran.
This is not only because none of the acts meets the threshold to allow for lawful use of force in self-defense; it is also because the United States is not the victim of these wrongs. Burns alleged that Iran is killing Americans in Iraq. But the U.S. is in Iraq at the invitation of the Iraqi leadership, namely Prime Minister Nouri al-Maliki. Attacks in Iraq, even on Americans, are still attacks against Iraq. Therefore, Iraq is the state with the legal authority to respond to such wrongs. Maliki, a Shi'a Muslim with close ties to Iran, seems very unlikely to take forceful countermeasures against Iran. But it is Maliki who has the authority to make such a decision, not President Bush. If the U.S. does not like how Maliki is responding to American deaths, its option is to pull Americans out of Iraq and sue Iraq for failing to make efforts within its capacity to protect Americans.
Indeed, in response to a United States military briefing in Baghdad on February 11 about alleged Iranian weapons shipments to Iraq, the Iraqi deputy foreign minister was quoted after the briefing in the Washington Post as saying, "'If they [the Americans] have anything really conclusive, then they should come out and say it openly, then we will pick it up from there and use diplomatic channels' to discuss it with Iranâ¦." 
It may be, however, that the United States is not really interested in responding to the wrongs it is complaining about, but rather is developing a pretext for attacking Iranian nuclear research sites. Iran may have aspirations to acquire nuclear weapons in violation of its obligations under the Nuclear Non-Proliferation Treaty.  While unlawful and open to countermeasures, the acquisition of nuclear weapons is not an armed attack giving rise to a right of self-defense. Thus, the U.S. simply has no right to take armed action against Iran for developing nuclear weapons.
So the U.S. may be talking up Iran's involvement in Iraq just now as an alternative basis for justifying an attack. It may not, however, justify bombing Iranian nuclear research sites in response to Iranian support for insurgent groups, incursions into Iraq, or similar conduct. If any of these actions gave rise to the right to use force in collective self-defense, the response would have to aim at ending the conduct giving rise to the right as required by the fundamental principles of necessity and proportionality. States may only use force if necessary to accomplish a lawful end and only that force proportionate in terms of the loss of civilian life and property to the value of the military objective. Even the Security Council is limited in what it can authorize with respect to military force under these principles. As the ICJ said in the Nuclear Weapons case, "'there is a specific rule whereby self-defence would warrant only measures which are proportional to the armed attack and necessary to respond to it, a rule well established in customary international law'. This dual condition applies equally to Article 51 of the Charter, whatever the means of force employed." 
Even if the development of nuclear weapons was a lawful trigger to armed self-defense, bombing will have little or no effect on the research program but would, in all likelihood, result in widespread civilian casualties and terrible destruction. In order to degrade Iran's nuclear program, the U.S. would have to bomb heavily populated areas all over the country. Yet, according to Newsweek, "the generals are convinced that no amount of firepower could do more than delay Tehran's nuclear program."  Killing and destroying in such a pointless exercise would clearly violate the restraints of necessity and proportionality.
Thus, any use of force against Iran by the United States based on a claim of individual or collective self-defense would be unlawful in the current circumstances. The Bush Administration has attempted to change the law as it applies to the U.S. in its National Security Strategies of 2002 and 2006, but one state alone cannot change the lawânot even the United States. The world community just re-affirmed the UN Charter rules on the use of force at the World Summit in September 2005, in effect rejecting the U.S. attempts to re-define the law. 
Some may argue that the international legal regime on self-defense is too restrictive, that it does not give the "good guys" enough options to defend themselves against rogue states with leaders like Ahmadinejad. But the "good guys" should be employing diplomacy, turning to armed self-defense only in situations involving significant armed attack. The proven strategy that works to end nuclear weapons programs is not force anyway, but diplomacy: South Africa gave up nuclear weapons and transformed itself into a nation that respects international law and human rights. Libya elevated its status almost overnight when it ended its nuclear weapons program. Making a generous offer to North Korea right now to give up its program could be very valuable toward persuading Iran to do the same.
By exercising diplomacy rather than engaging in the unlawful use of force, the U.S. would go a long way toward convincing the international community that it has the best interest of the Middle East in its plans, not another war.
 Gates: U.S. Can Prove Iran's Iraq Role, ASSOC. PRESS, Feb. 9, 2007 (available at http://www.nytimes.com/aponline/world/AP-Gates.html). See also, Michael Hirsh and Maziar Bahari, Rumors of War, NEWSWEEK, Feb. 19, 2007 (available at http://www.msnbc.msn.com/id/17086418/site/newsweek). (The article discusses the rumors of war and includes a denial by Under Secretary of State Burns that the U.S. is planning a war.)
 The resolution that was passed is UN S.C. Res. 1696 (2006).
 Transcript of Interview with Under Secretary of State Nicholas Burns, Morning Edition, NPR RADIO (available at http://www.npr.org/templates/story/story.php?storyId=7107364).
 Military and Paramilitary Activities (Nicar. v. U.S.), 1986 I.C.J.14,103 (June 27)(emphasis added)[hereinafter "Nicaragua"]. See also, MARY ELLEN O'CONNELL, INTERNATIONAL LAW AND THE USE OF FORCE, chap. 7 (2005).
 Nicaragua at 102-04. See also, Eritrea-Ethiopia Claims, Partial Award, Jus Ad Bellum, Ethiopia's Claims 1 8, Dec. 19, 2005, www.pca-cpa.org and INTERNATIONAL INCIDENTS: THE LAW THAT COUNTS IN WORLD POLITICS (W. Michael Reisman & Andrew R. Willard eds., 1988).
 Nicaragua at 110-11.
 Id. at 120-21.
 CHRISTIAN GRAY, INTERNATIONAL LAW AND THE USE OF FORCE 133 (2000).
 See, e.g., Hirsh and Bahari, supra note 1.
 Michael Rosen, How to Get Justice for Ahmadinejad, TCS DAILY, Jan. 12, 2007 (available at http://www.tcsdaily.com/article.aspx?id=011207B).
 While Iran has not complied with the Security Council resolution to end its program of uranium enrichment, the IAEA has issued no new assessment since its report last February finding no conclusive proof that Iran was engaged in production of nuclear weapons. IAEA, Implementation of the NPT Safeguards Agreement in the Islamic Republic of Iran, at 5, IAEA Doc. GOV/2006/15, (February 27, 2006) (Report by the Director General). Elaine Sciolino, U.N. Agency says Iran Falls Short on Nuclear Data, N.Y. TIMES, April 29, 2006, at A1.
 Joshua Partlow, Military Ties Iran to Arms in Iraq, Feb. 12, 2007, WASH. POST, at A01. Regarding border incursions, again, Iraqi officials have said they will handle this problem diplomatically. See, John F. Burns, Iran and Iraq to Join to Seal Borders Against Insurgents, N.Y. TIMES, May 28, 2006, at 14. Burns reports that Iran and Iraq plan to form a joint commission to control their borders and to thwart the efforts of groups threatening the security of the two nations.
 Difference over how to punish Iran's nuclear defiance strain international alliance, ASSOC. PRESS, Feb. 8, 2007 (available at http://www.iht.com/articles/ap/2007/02/08/europe/EU-GEN-Squeezing-Iran.php).
 Legality of the Threat or Use of Nuclear Weapons, Advisory Opinion, 1996 I.C.J. 226, para. 48 (Jul. 8) (citing the Nicaragua Case, supra note 4, at para. 176). See also Oil Platforms (Iran v. U.S.) 2003 I.C.J. 151, 198 (Nov. 6).
 Hirsh and Bahari, supra note 1.
 G.A. Res. 60/1, U.N. Doc. A/RES/60/1 (Oct. 24, 2005).
Mary Ellen O'Connell holds the Robert and Marion Short Chair in Law at the University of Notre Dame