ISRAEL NOW faces the biggest-ever challenge to its monopoly on the bomb in the Middle East from Iran. For Israel, Tehran is a dangerous opponent, close and threatening. There is a virtually unanimous consensus in Israel that Iran cannot be allowed to acquire nuclear weapons. From left to right, Israelis see an existential threat to their very survival. Current Prime Minister Benjamin Netanyahu argued at the Brookings Institution’s Saban Forum in Jerusalem in 2007 that Iran is a “crazy,” even suicidal, state that will be prepared to sacrifice millions of its own citizens in a nuclear exchange with Israel.

Though other Israeli leaders are more cautious, even they are strongly determined to keep Israel’s monopoly on nuclear weapons. Ephraim Sneh, former deputy defense minister and a much-decorated retired general in the Israel Defense Forces (IDF), notes that “the most salient strategic threat to Israel’s existence is Iran.” They fear Israel’s strategic room for maneuver in the region would be constrained by an Iranian nuclear deterrent. The success of Hezbollah and Hamas in the last few years has only added to Israeli concern.

It is clear from statements of Israeli military and intelligence officials and numerous press leaks that planning for a military operation to prevent Iran from acquiring nuclear weapons is well under way in Israel. Defense Minister Ehud Barak has said that “the things that we do behind the scenes, far from the public eye, are far more important than the slogan charade,” implying that Israeli covert capabilities are already hard at work trying to cope with the Iranian threat, and preparing to attack it if they must. It is impossible to know what those plans entail in detail without access to the IDF’s classified documents, but Israelis say the mission is not an impossible one.

It is certainly a challenging one. Distance alone makes Iran a much more difficult target than Iraq or Syria. The most direct route from Israel to Iran’s Natanz facility is roughly 1,750 kilometers across Jordan and Iraq. The alternatives via Turkish airspace (over 2,200 kilometers) or Saudi airspace (over 2,400 kilometers) would also put the attack force into the skies of American allies equipped with American fighter aircraft. Moreover, unlike Iraq and Syria, but like Pakistan, the Iranian program is dispersed throughout several facilities and sites around the country, some of which are underground and hardened. An attack might require multiple missions over several days. Israel would have to assume some aircraft and pilots would be lost.

Though Israel is giving diplomacy and sanctions time to change Iranian behavior, few in Jerusalem expect the soft approach to work. Most also doubt the United States will use force. America already is engaged in two wars in the Middle East, and all the disadvantages of an Israeli attack apply to an American one as well. To keep its monopoly on the bomb Israel may well choose to strike.

AN ISRAELI attack on Iran is a disaster in the making. And it will directly impact key strategic American interests. Iran will see an attack as American supported if not American orchestrated. The aircraft in any strike will be American-produced, -supplied and -funded F-15s and F-16s, and most of the ordnance will be from American stocks. Washington’s $3 billion in assistance annually makes possible the IDF’s conventional superiority in the region.

Iran will almost certainly retaliate against both U.S. and Israeli targets. To demonstrate its retaliatory prowess, Iran has already fired salvos of test missiles (some of which are capable of striking Israel), and Iranian leaders have warned they would respond to an attack by either Israel or the United States with attacks against Tel Aviv, U.S. ships and facilities in the Persian Gulf, and other targets. Even if Iran chooses to retaliate in less risky ways, it could respond indirectly by encouraging Hezbollah attacks against Israel and Shia militia attacks against U.S. forces in Iraq, as well as terrorist attacks against American and Israeli targets in the Middle East and beyond.

America’s greatest vulnerability would be in Afghanistan. Iran could easily increase its assistance to the Taliban and make the already-difficult Afghan mission much more complicated. Western Afghanistan is especially vulnerable to Iranian mischief, and NATO has few troops there to cover a vast area. President Obama would have to send more, not fewer, troops to fight that war.

Making matters worse, considering the likely violent ramifications, even a successful Israeli raid would only delay Iran’s nuclear program, not eliminate it entirely. In fact, some Israeli intelligence officials suspect that delay would only be a year or so. Thus the United States would still need a strategy to deal with the basic problem of Iran’s capabilities after an attack, but in a much more complicated diplomatic context since Tehran would be able to argue it was the victim of aggression and probably would renounce its NPT commitments. Support for the existing sanctions on Iran after a strike would likely evaporate.

The United States needs to send a clear red light to Israel. There is no option but to actively discourage an Israeli attack. There is precedent for Washington telling Israel not to use force against a military threat. In the 1991 Gulf War, President George H. W. Bush pressed Israeli Prime Minister Yitzhak Shamir not to target Iraqi Scud missile launchers that were attacking Israel. Most importantly, Bush refused to give the Israelis the iff codes (encrypted signals to identify aircraft as “friend or foe”) and approval to enter Iraqi airspace, thereby indicating that Israeli aircraft would be flying into harm’s way. Israel’s preferred option of a limited ground-force incursion into western Iraq was also turned down. Of course, in 1991 we were at war with Iraq and committed to stepping up our own attacks on Iraqi Scuds, but the point remains—America does have influence and it should be wielded.

PERSUADING ISRAEL not to attack Iran really means convincing Israel that now is the time to give up its regional nuclear monopoly. If we are going to do so, that means enhancing Israel’s deterrence posture. This is the only way Israel can feel (and will be) safe from an Iranian nuclear threat.

Iran will be subject to the same deterrence system that other nuclear-weapons states have accommodated since 1945: it will try to use its nuclear status to intimidate non-nuclear-weapons nations but will avoid conflict that could escalate into an atomic exchange with another nuclear power. Without doubt, throughout its history the Islamic Republic has behaved very disagreeably, but it has been careful to avoid taking actions that would lead to catastrophic consequences.

In the defining event of modern U.S.-Iran relations, for example, the hostage crisis of 1979–1981, Tehran acted in ways that were in clear violation of international law, but when it perceived a given course would provoke a massive violent American response, it desisted. In the summer of 1980 Iranian leaders repeatedly threatened to put the American hostages on trial for espionage. President Jimmy Carter made clear that any trials would produce a military response and Iran retreated. In the 1988 undeclared naval war in the Persian Gulf between the United States and Iran over reflagged Kuwaiti tankers, Iran attacked U.S. Navy ships but was careful to keep the conflict from escalating into a full-scale war. When the USS Vincennes inadvertently shot down an Iran Air civilian airliner, Ayatollah Khomeini sensed the conflict was getting out of control and agreed to a cease-fire with Iraq and the United States.

Throughout the Iran-Iraq war, Tehran also chose to avoid actions that would cross WMD thresholds. It was Iraq that first used chemical weapons on the battlefield, not Iran, and it was Iraq that first used missiles against Iranian cities. In the mid-1990s, when the United States determined Iran was behind the terrorist attack on the U.S. Air Force barracks at Khobar, Saudi Arabia, and warned Iran that any further attacks would prompt military retaliation, Iran desisted from carrying out operations on American military facilities in the Gulf and elsewhere. Today Iran is careful to limit its support of anti-American insurgents in Iraq and Afghanistan to low-intensity conflict and asymmetric warfare to avoid a major American military response. Contrary to Netanyahu’s cries, Iran is not a crazy state. A nuclear security guarantee to Israel, if backed by a credible arsenal, will deter Tehran.

IN SUCH a dire (but manageable) situation, the United States needs to bolster Israel’s capabilities now. The administration should take another look at extending the American nuclear umbrella. It is an idea that has long been floated. At the Camp David summit in 2000, then–Israeli Prime Minister Ehud Barak first raised the idea of a U.S.-Israeli mutual-defense treaty to provide Israel with a nuclear guarantee against Iran while sitting in a meeting with then-President Bill Clinton and two note takers (me and an Israeli). Clinton was positive about the idea if the summit succeeded. The proposal died when the Israeli-Palestinian peace process collapsed. But it is worth taking another look. And it is a policy prescription not too difficult to employ.