If Israel should launch a unilateral attack on Iran in response to a mere threat to weaponize its nuclear program, should the United States come to Israel’s aid?

Before responding affirmatively, American policymakers should consider a word that has rarely appeared in the public debates on Iran or in the media. That word is 'law': international law and US law.

During the meeting of the pro-Israel lobbying group, AIPAC, earlier this month, Republican senators and presidential candidates fell over themselves in calling for US solidarity with Israel should it launch a preemptive strike on Iran.

Senate Minority Leader Mitch McConnell promised to introduce authorization for the use of “overwhelming military force” against Iran if American intelligence shows that Tehran has decided to build a nuclear weapon or it has started to enrich uranium to weapons-grade level. In a March 5 Washington Post op-ed, Mitt Romney hinted that as president he would use military force if Iran acquires a nuclear weapon.

In his remarks to the pro-Israel gathering, President Obama observed that “there is too much loose talk of war” and stated his preference for diplomacy. Yet he felt compelled to declare “I will take no options off the table,” including “a military effort.” Absent from his speech was a clear statement of what it would take to trigger such an effort. What “red line” would have to be crossed?

In his address to AIPAC, Israeli Prime Minister Benjamin Netanyahu expressed his impatience with diplomacy and asserted Israel’s right “to defend itself, by itself, against any threat.”

For all the talk in Washington of solidarity with Israel and for all the saber rattling on Iran, jumping to Israel’s aid in a preemptive strike would be a clear violation of international and US law.

Article 51 of the United Nations Charter acknowledges the right of a member state to take military action in “self-defense” when responding to “an armed attack.” The charter does not, however, justify preemptive attacks in response to a buildup of military capacity (nuclear or otherwise) or much less a response to mere hostile intent. When the Senate ratified the UN Charter as a treaty in 1945, Article 51 became a part of US law by the terms of Article VI of the US Constitution.

In spite of these legal norms, President George W. Bush launched his shock and awe attack on Iraq in 2003 based on an assumption (later proved wrong) that the country harbored weapons of mass destruction. In doing so, he sent a powerful message that any nation should be allowed to attack another on the ground of presumed military capacity and/or hostile intent. Article 51 suffered a severe blow along with the citizens of Baghdad.

In a recent Los Angeles Times op-ed, Yale Law professor Bruce Ackerman reminds readers that the US joined in a UN Security Council condemnation of Israel’s preemptive strike on an Iraqi nuclear reactor in 1981, and that even the 1962 Cuban missile crisis failed to provoke a preemptive attack by the US.

If American adherence to Article 51 is to be restored, Mr. Obama should refuse to follow the precedent established by his predecessor, Mr. Bush. If the president is to honor his oath-of-office pledge to uphold the Constitution, he should refuse to be drawn into a war on Iran based largely on Israeli fears. Nuclear capacity and hostile intent fall far short of “armed attack.”

Every country has a vital stake in respecting the shared norms of international law. Without legal norms, violence has no limit. An America that okays preemptive strikes today opens itself to such attacks by other countries tomorrow.

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Unless Obama clarifies in advance that only Israeli self-defense against an armed attack would trigger American participation, Americans risk being swept into another conflict that would violate both international and domestic law. The Israeli government and the American public should know where the US president draws the line, whatever color he gives it.

L. Michael Hager is the co-founder and former director general of the International Development Law Organization.