Wednesday’s IAEA report offered more proof than ever that Iran is racing toward a nuclear weapon. Even President Obama acknowledges this fact. Yet Obama has refused to endorse the tough approach advocated by Israeli Prime Minister Netanyahu. The reason is simple: Israel faces an existential threat of an Iranian nuclear attack, while the United States does not.

With diplomacy and sanctions failing, world leaders still give lip service to the dangers of a nuclear Iran. But they often dismiss the idea that Iran would actually use a nuclear weapon against Israel. They believe that Iran is a rational actor, and that Israel’s strong nuclear deterrent is sufficient to safeguard the Jewish state. It is not.

Israel’s deterrent capacity is only effective against conventional nuclear attacks. If Iran’s Revolutionary Guard, or one of its terror proxies, detonated a suitcase bomb inside of Israel, it would be nearly impossible to prove that Iran’s leaders ordered the attack. And without conclusive evidence of Tehran’s direct involvement, an Israeli counterattack would be illegal, unjust, and unwise. Therefore, it is plausible that Iran could use a tactical nuclear weapon against Israel without a serious fear of an Israeli reprisal.

Many in the West dismiss this threat out of hand. They argue that the principle of “Mutually Assured Destruction” (MAD) protects Israel: If Iran were to fire a nuclear weapon at Israel, Israel would retaliate and destroy Iran.

On the face of it, MAD appears logical. Iran’s leaders are not suicidal (in fact, their main purpose in pursuing a bomb is self-preservation), so Israel’s nuclear arsenal seems like a strong deterrent. But MAD only works if Tehran launches a conventional, traceable, and undisputed nuclear attack on Israel (for example, via a ballistic missile launched from inside Iran).

If, however, Iran were to provide a small nuclear weapon to Hamas or Hezbollah, or use the Revolutionary Guard to detonate a nuclear device in Israel, MAD would no longer apply.

Israel could only launch a counterattack if it had conclusive proof that the nuclear attack was ordered by Iran’s leaders. If Tehran fired a nuclear warhead via a missile silo in Iran, culpability would be fairly easy to prove. It would strain credibility for the mullahs to argue that such a strike occurred without its direction. Further, it is a principle of the laws of war that a country’s leaders are responsible for the actions of their military officers.

But if Iran smuggled a nuclear weapon to a terror proxy, or used the Revolutionary Guard to covertly detonate a device in Israel, proving culpability might be impossible.

Israel would first need to establish that the nuclear device came from Iran, not the former Soviet Union or North Korea. But if the device is successfully detonated, obtaining this proof would be extremely difficult according to scientists at the U.S. National Institute of Standards and Technology.

But even if Israel could trace the device back to Iran, it would still need to prove that Tehran itself ordered the detonation. This could be impossible to prove with certainty.

How, for example, could Israel prove that the Ayatollah gave the bomb to Hezbollah — not that Hezbollah stole the bomb, or that a radical segment of the Revolutionary Guard smuggled it into Israel without permission? Yes, Israel could use human intelligence to cast doubt on such a claim, but would this be enough to justify starting a nuclear war?

Similarly, even if Israel could definitively prove that the bomb was set off by a Revolutionary Guard operative, Iran’s leaders could claim that the operative was working on his own. True, Israel could invoke the principles of international law to hold Tehran accountable, but in a nebulous situation like this, would that principle be enough to justify a possible nuclear war?

Finally, even if Israel traced the transfer of the bomb from Iran to Hezbollah or Hamas, Iran’s leaders could still claim that it was done without their permission. Again, the claim might be dubious, but without definitive proof, Israel might be unwilling to launch a counterattack.

And without definitive proof of Tehran’s involvement, an Israeli counterattack would be illegitimate and foolish. How could Israel legally punish Iran for an act that it can’t prove its leadership was responsible for? And how would an Israeli strike serve as a deterrent to future action, when Israel can’t prove that the first attack came from Iran?

An Israeli counterattack would also give Iran a legitimate excuse to use its nuclear arsenal against Israel. And if Israel couldn’t prove that Iran’s leaders ordered the initial attack, Israel would probably have no international support for a counterattack or the war that would follow.

Critics might argue this premise is implausible: that Iran would never trust a nuclear weapon to its terror proxies or try to carry out such an attack via the Revolutionary Guard. But Iran has shown a willingness to transfer a wide range of lethal weapons and technologies to its terror proxies, and a willingness to attack Israel on foreign soil.

Iran has a long history of arming foreign groups and terror proxies, most recently in Syria. The United States has publicly designated Iran as the world’s leading state sponsor of terrorism. According to the State Department, the Revolutionary Guard has provided arms and funding to the Taliban, Assad’s regime in Syria, as well as Hezbollah and Hamas.

In recent months, Iran has been caught sending weapons to terrorists in Yemen and ammunition to despotic regimes in Africa. Iran was also responsible for the recent slaughter of Israeli tourists in Bulgaria, as well as the targeting of Israeli diplomats in Thailand, Georgia, and India.

And, most notably, Iran has transferred heavy weapons to Hamas and Hezbollah for use against Israel. The head of Iran’s Revolutionary Guard publicly boasted about providing long-range rockets to Palestinian terrorists during the November 2012 Gaza conflict, and according to reports, Revolutionary Guard forces went to Gaza to help Hamas launch these rockets.

And Hezbollah, with Iranian assistance, recently flew its first drone surveillance mission into Israel. Hezbollah is also stockpiling an arsenal of 50,000 missiles that are capable of striking Israel, and many of these came from Iran.

Given this track record, and the fact that Iran’s leaders don’t believe Israel has a right to exist, it is at least possible that Iran would launch a nuclear attack if the regime believed it could get away with it.

It’s also possible that if the Ayatollah’s regime collapsed, the mullahs might lose control over their nuclear arsenal or order the Revolutionary Guard to launch a nuclear attack against Israel as a parting blow. This is a real threat that Israel, the U.S., and the international community must consider as the crisis comes to a boiling point.

This is not to suggest that Israel must launch a military strike on Iran’s nuclear program. (In fact, the only sure way to prevent a nuclear Iran may be regime change, and an Israeli strike might have the undesirable effect of rallying the Iranian people around their government.) But the danger that Iran will detonate a nuclear bomb in Israel is something that Israeli policymakers must consider in deciding how to deal with the Iranian nuclear program.

America, unlike Israel, does not face an existential threat from Iran. And if the United States refuses to act, Israel must decide what to do based on its own interests, just as Americans expect President Obama to make his decision based on America’s interests.

David Meyers served in the White House from 2006 to 2009, and later in the United States Senate. He is currently pursuing graduate studies at Columbia University.