The past fortnight has witnessed an unprecedented, open public debate in the Israeli media about whether, and when, to attack Iran's nuclear installations. While in the past six decades, Israel's wars have often been followed by debates about this or that political or military aspect, or even the justice of a given war (vide Israel's Sinai Campaign in 1956 and Israel's invasion of southern Lebanon in 1982), none have ever been preceded by major, principled, open discussion.

Nearly all Israeli Jews are agreed that a nuclear-armed Iran, with its fanatical, messianic Islamist leaders, represents a mortal threat to Israel's existence, and all—like all Western intelligence agencies—are agreed that Iran is bent on attaining nuclear weapons as quickly as possible. Some observers believe the Iranians will halt their project just short of actually producing the bombs, leaving a few screws still to be turned; I personally doubt this on the view that Iran wants a nuclear arsenal in order to project power, overawe its neighbors, deter would-be assailants and perhaps destroy Israel.

No one in Israel took seriously the American intelligence assessment of 2007 that Iran had halted its weapons production effort in 2003, and it appears that no one in Washington today adheres to that assessment. The International Atomic Energy Agency is expected this week to publish a new report putting the final nail in the coffin of that assessment. Parts of that report were leaked to the Washington Post on Monday, and they reveal that Iran has mastered the procedures needed to build a nuclear weapon in the near future. Indeed, it seems clear that the Iranians are working at breakneck speed on nuclear weaponization models, enrichment facilities and nuclear-capable delivery systems. The Russians and Chinese are understood to be trying to postpone the report's publication and/or to water down its conclusions in order to propitiate their friends in Teheran, again displaying strategic short-sightedness (not to say immorality).

The internal Israeli debate was launched in an article by Nahum Barnea, perhaps Israel's most important journalist, in the country's most popular daily, Yediot Aharonot, a fortnight ago. He briefly described the split in the Israeli hierarchy between those advocating an assault now on Iran's nuclear facilities—Prime Minister Binyamin Netanyahu and his defense minister, Ehud Barak—and those either opposed in principle to such an assault or who believe there is still time for diplomacy and sanctions to effect a halt to the Iranian program. The discord cuts across party lines and is not between traditional hawks and doves. There seems to be agreement that the Iranians are still a year and a half to two years away from the bomb, and some of Netanyahu's and Barak's opponents believe that Israel cannot, under any condition, mount an assault against the Iranian facilities without a bright green light from Washington.

But Washington has made it amply clear, most recently during Defense Secretary Leon Panetta's visit to Jerusalem last month, that it opposes unilateral Israeli military action. Commentators have said that Netanyahu and Barak declined to give Panetta an ironclad assurance that Israeli would not act alone. Barak reportedly said that when the issue is Israel's existence, Israel will do what needs to be done even if alone, whatever the views of Israel's friends abroad.

At the same time, Netanyahu and Barak continue to insist that the Iranian bomb is a global and regional threat, not merely a threat to Israel—and as such needs to be neutralized by the international community, not by Israel alone. But both men realize that the United States, after long years of relatively unsuccessful and certainly indecisive battle against Muslims in Afghanistan and Iraq (and, to a degree, Pakistan), has lost the stomach for another, possibly major, fight. President Obama may yet surprise everyone—and recent reports from London indicate that Britain may be ready to join an American-led assault on Iran, should one be launched—but no one in Israel is banking on or can bank on an American attack against the Iranian facilities. And the Iranians are daily drawing closer to the bomb.

The problems facing Netanyahu and Barak are acute. To begin with, there is strong internal opposition to an Israeli strike, and leaders of democracies are loath to go to war without a popular mandate. Leading the opposition to an attack now are reportedly the heads of the Israeli Defense Force (IDF), the General Security Service (the Shin Bet) and the Mossad, and a number of senior right-wing politicians who sit on the country's top decision-making body, the eight-man Inner Cabinet, including Moshe Yaalon, a former IDF chief of staff, and Dan Meridor, the well-respected minister in charge of the country's secret services.

Another important opponent of war now is former IDF chief of staff and defense minister Shaul Mofaz, the chairman of the Knesset's key Foreign Affairs and Defense Committee. In an interview last weekend, he told Nahum Barnes:

The danger in attacking Iran is not in the attack itself, but in what will happen in the region on the day after. A regional war is likely to break out, which from our perspective will be an existential conflict. The operation may succeed [i.e., the air force attack on the Iranian nuclear facilities]—but the patient [Israel] may die . . . Strategically speaking . . . without American leadership in the battle against the Iranian nuclear [project], without America's support and involvement, we will be taking an unreasonable risk.

Mofaz may obliquely have been hinting at the fact that there is no knowing, in advance, how successful an Israeli air and missile assault will be (and, given the nature of the Iranian regime and the secrecy surrounding the nuclear project, it is possible that, in the wake of an assault, the amount of damage actually inflicted will not be fully known by Israel and the West): Will Israel succeed in completely destroying the Iranian nuclear facilities and halting the bomb project, or will its assault only cause damage leading to a postponement of bomb production by a year or two? Without doubt, such an assault will energize the Iranians to push ahead at full throttle to achieve nuclear bombs—and will, in addition, provide them with a measure of post facto justification in the eyes of many in the international community for their nuclear project.

But this is only one problem. More significant still will be the problem of the "day after,” as Mofaz put it. It boils down to the likelihood that such a strike will trigger an open-ended, multi-faceted regional war in which Iranian, Hezbollah (Lebanon) and Hamas (Gaza Strip) rockets, in their hundreds, daily, will rain down on Israel's cities, including Tel Aviv and Jerusalem. Iran over the years has armed its Lebanese and Palestinian proxies with literally tens of thousands of rockets, which, when launched en masse, could paralyze Israel for weeks or even months. And such a missile onslaught is likely to be accompanied by a worldwide terrorist assault on Israeli, Jewish and Western targets (embassies, synagogues and community centers, military bases, industrial plants and infrastructure, etc.) by Iranian agents and sympathizers. Islamists are likely to believe that Israel acted with American approval or complicity, whatever denials emerge from Washington and Jerusalem.

The 2006 Israeli assault on Hezbollah in southern Lebanon and the 2009 attack on Hamas in the Gaza Strip demonstrated that rocketry couldn’t be neutralized by air power alone. In the case of Hezbollah, the organization continued throughout to rain down on Israel a daily dose of two hundred missiles despite the month-long aerial pounding. This means that in the next war, Israel would need to fully mobilize its reserves and conquer and occupy in ground invasions southern (and possibly central) Lebanon and the Gaza Strip to end the missile barrages (though it is unlikely, given the distances involved, that Israel will be able, using only conventional weaponry, to neutralize the expected Iranian missile attacks on Israel's cities).

And, as if this were not enough, such a regional conflagration is likely to affect, and possibly suck in, Egypt (a growingly Islamist country) and Syria (an Iranian ally led by a regime that would be only too happy to divert its people's attention away from their internal problems) and possibly other players, including Israel's own Arab minority. Without doubt, all of this would have a significant, negative impact on Israel—its economy, social cohesion, etc.—for months and years to come, whatever the outcome of the hostilities (which are not likely to result in a clear decision).

It is no wonder then that Israel's decision makers are locked in heated debate over what may turn out to be the most important decision taken in Jerusalem since the establishment of the Jewish state.

Benny Morris is a professor of history in the Middle East Studies Department of Ben-Gurion University of the Negev. His most recent book is One State, Two States: Resolving the Israel/Palestine Conflict (Yale University Press, 2009).