When is it not...but this article doesn't paint a pretty picture.
http://www.theatlantic.com/magazine/print/2010/09/the-point-of-no-return/8186/Its a long article, so I won't post the whole thing, but I recommend reading it.
Quote:
IT IS POSSIBLE that at some point in the next 12 months, the imposition of devastating economic sanctions on the Islamic Republic of Iran will persuade its leaders to cease their pursuit of nuclear weapons. It is also possible that Iran’s reform-minded Green Movement will somehow replace the mullah-led regime, or at least discover the means to temper the regime’s ideological extremism. It is possible, as well, that “foiling operations” conducted by the intelligence agencies of Israel, the United States, Great Britain, and other Western powers—programs designed to subvert the Iranian nuclear effort through sabotage and, on occasion, the carefully engineered disappearances of nuclear scientists—will have hindered Iran’s progress in some significant way. It is also possible that President Obama, who has said on more than a few occasions that he finds the prospect of a nuclear Iran “unacceptable,” will order a military strike against the country’s main weapons and uranium-enrichment facilities.
But none of these things—least of all the notion that Barack Obama, for whom initiating new wars in the Middle East is not a foreign-policy goal, will soon order the American military into action against Iran—seems, at this moment, terribly likely. What is more likely, then, is that one day next spring, the Israeli national-security adviser, Uzi Arad, and the Israeli defense minister, Ehud Barak, will simultaneously telephone their counterparts at the White House and the Pentagon, to inform them that their prime minister, Benjamin Netanyahu, has just ordered roughly one hundred F-15Es, F-16Is, F-16Cs, and other aircraft of the Israeli air force to fly east toward Iran—possibly by crossing Saudi Arabia, possibly by threading the border between Syria and Turkey, and possibly by traveling directly through Iraq’s airspace, though it is crowded with American aircraft. (It’s so crowded, in fact, that the United States Central Command, whose area of responsibility is the greater Middle East, has already asked the Pentagon what to do should Israeli aircraft invade its airspace. According to multiple sources, the answer came back: do not shoot them down.)
In these conversations, which will be fraught, the Israelis will tell their American counterparts that they are taking this drastic step because a nuclear Iran poses the gravest threat since Hitler to the physical survival of the Jewish people. The Israelis will also state that they believe they have a reasonable chance of delaying the Iranian nuclear program for at least three to five years. They will tell their American colleagues that Israel was left with no choice. They will not be asking for permission, because it will be too late to ask for permission.
When the Israelis begin to bomb the uranium-enrichment facility at Natanz, the formerly secret enrichment site at Qom, the nuclear-research center at Esfahan, and possibly even the Bushehr reactor, along with the other main sites of the Iranian nuclear program, a short while after they depart en masse from their bases across Israel—regardless of whether they succeed in destroying Iran’s centrifuges and warhead and missile plants, or whether they fail miserably to even make a dent in Iran’s nuclear program—they stand a good chance of changing the Middle East forever; of sparking lethal reprisals, and even a full-blown regional war that could lead to the deaths of thousands of Israelis and Iranians, and possibly Arabs and Americans as well; of creating a crisis for Barack Obama that will dwarf Afghanistan in significance and complexity; of rupturing relations between Jerusalem and Washington, which is Israel’s only meaningful ally; of inadvertently solidifying the somewhat tenuous rule of the mullahs in Tehran; of causing the price of oil to spike to cataclysmic highs, launching the world economy into a period of turbulence not experienced since the autumn of 2008, or possibly since the oil shock of 1973; of placing communities across the Jewish diaspora in mortal danger, by making them targets of Iranian-sponsored terror attacks, as they have been in the past, in a limited though already lethal way; and of accelerating Israel’s conversion from a once-admired refuge for a persecuted people into a leper among nations.
If a strike does succeed in crippling the Iranian nuclear program, however, Israel, in addition to possibly generating some combination of the various catastrophes outlined above, will have removed from its list of existential worries the immediate specter of nuclear-weaponized, theologically driven, eliminationist anti-Semitism; it may derive for itself the secret thanks (though the public condemnation) of the Middle East’s moderate Arab regimes, all of which fear an Iranian bomb with an intensity that in some instances matches Israel’s; and it will have succeeded in countering, in militant fashion, the spread of nuclear weapons in the Middle East, which is, not irrelevantly, a prime goal of the enthusiastic counter-proliferator who currently occupies the White House.
He paints a pretty convincing picture.