Imagine a moment when President Obama has only two alternatives: prepare to live with a nuclear-armed Iran or embark on the perilous path of military action to stop it.

Imagine that diplomacy has run its course, after prolonged and inconclusive negotiations; that surging international oil prices have undercut the power of economic sanctions against Tehran; and that reliable intelligence says the Islamic republic's weapons program is very close to reaching its goal.

Facing such conditions, would Obama use force against Iran?

Former CIA chief Michael Hayden believes such a move would be necessary, recently telling CNN that a U.S. military strike against Iranian facilities "seems inexorable" because diplomacy is failing. "We engage. They continue to move forward," Hayden warned. "We vote for sanctions. They continue to move forward. We try to deter, to dissuade. They continue to move forward."

Obama has also emphasized Tehran's own actions as the determining factor in a U.S. response. "We offered the Iranian government a clear choice," he said on July 1, when he signed the Iran Sanctions Act. "It could fulfill its international obligations and realize greater security, deeper economic and political integration with the world . . . or it could continue to flout its responsibilities and face even more pressure and isolation."

And a few days later, the president stressed in an interview with Israeli television that although his administration will "continue to keep the door open for a diplomatic resolution . . . I assure you that I have not taken options off the table."

As a practical matter, however, Obama's decision on the use of force would hinge on factors well beyond Iran's timetable for obtaining a bomb. In fact, the political, military and policy constraints Obama would face could compel his administration to forgo the military option no matter how close Iran gets to joining the nuclear club.

First, there is the United Nations to consider. Both the George W. Bush and Obama administrations have centered their responses to Iran's nuclear ambitions on the Security Council, implying that Iran's conduct violates not just U.S. national security interests but also Tehran's international legal obligations. For Obama, the United Nations has doctrinal centrality, as well. According to the president's recently released National Security Strategy, "When force is necessary . . . we will seek broad international support, working with such institutions as NATO and the U.N. Security Council."

Given this impulse to multilateralize the use of force and link it to the rule of law as well as to self-interest, the administration would have a hard time attacking Iran without Security Council backing. This particular high ground, however, might be unattainable. Indeed, the United States has obtained a series of U.N. resolutions censuring Iran not because its legal arguments and foreign policy views have wowed the world, but simply because its European partners have feared that Washington might otherwise take matters into its own hands. These anxieties were more acute during the Bush years, but they have hardly dissipated with new occupants in the White House. From Europe's perspective, the U.N. process is designed not just to pressure Iran but also to enmesh the United States in cumbersome proceedings that limit its choices.

It may be comforting for Washington to blame China and Russia as the key obstacles to more forceful measures against Iran, but Britain and France -- where public opinion is already against participation in the war in Afghanistan -- also have little appetite for striking. An Obama team that has prioritized repairing ties with "old Europe" and resetting relations with Russia would have to think twice before putting these refurbished relationships at risk by bombing Iran. (A president who meticulously rehabilitated America's standing in Europe would scarcely be eager to don Bush's mantle as the Ugly American.)

Whatever progress Iran may make toward weapons of mass destruction, European diplomats and statesmen are likely to parade to Washington, concede America's concerns, affirm its intelligence findings -- and reject its policy recommendations. The United States would be advised to be patient and restock its economic sanctions kit for one more run at Tehran. In private, many strategists would summon their inner George Kennan and advise Washington that containment has worked with more powerful and unpredictable tyrants and can surely handle cautious mullahs and their rudimentary weapon. Washington would have to choose between an international coalition pledging rigorous containment of Iran, and the lonely, unpopular path of taking military action lacking allied consensus.

Domestic consensus would be critical as well. One of the tragedies of American history is that presidents have too often entangled the country in conflicts without forthright conversation with the public. Franklin Roosevelt and Lyndon Johnson engaged in their share of measured mendacity as they plunged the United States into very different wars. More recently, Bush's decision to preemptively invade Iraq was characterized by exaggerated threats and faulty information.