An Israeli raid on Iran's nuclear facilities would deliver a painful shock to the global economy, revive flagging Islamist militancy and possibly drag the United States into a regional war whether it backed its ally's attack or not.
As if that prospect was not alarming enough, any doubts Tehran entertained about the wisdom of building a nuclear weapon would vanish the moment the strike occurred.
These longstanding U.S. and European assumptions about the consequences of an Israeli attack on Iran are being re-examined with greater urgency in Western capitals after repeated warnings by Israel that the chance of a peaceful resolution may be closing.
There is concern that Israel may attack in coming months to disrupt the transfer of parts of Iran's nuclear development work to an underground site south of Tehran that may be invulnerable to conventional bombing.
Western experts want Israel to think long and hard before embarking on a raid many suspect would spark a broader conflict.
"Whoever attacks Iran's nuclear infrastructure is really making the decision to go to war with Iran," Richard Burt, a former chief U.S. negotiator at strategic arms reduction talks, told Reuters on the sidelines of the Munich Security Conference.
"We are talking about a range of great uncertainties, all of them basically negative, so this is one reason why consistently the joint chiefs of staff of the uniformed military in the U.S. do not like the idea of attacking Iran."
Analysts detect a growing gap between Israeli and Western views on using force against Tehran's nuclear programme, which Tehran says is purely for peaceful purposes but the West suspects is aimed at acquiring a nuclear weapons capability.
The New York Times recently reported that Israeli leaders, based on intelligence estimates and academic studies, had taken the view that an Israeli attack on Iranian nuclear installations would not produce such catastrophic events as regional war, widespread attacks by militants and massive oil price rises.
The newspaper said that Israeli leaders and agencies believe that Iran's threats to retaliate against Israeli and Western targets if attacked were "overblown" and partly bluff.
Asked to spell out the consequences of a strike, Israeli Deputy Foreign Minister Danny Ayalon said that he not want to explore a hypothetical question, arguing any risks "would be dwarfed in comparison to the danger of a nuclear Iran".
"One thing is clear," he told Reuters. "If Iran becomes nuclear then it's the end of world order as we know it ... This is what we have to think about, and not about what will happen in case some action is being taken.
US newspaper The Washington Post reported last week that Defense Secretary Leon Panetta believed Israel was likely to bomb Iran within months to stop it building a nuclear bomb.
Panetta declined to comment. But his alleged remarks and other Obama administration statements indicate the White House is focused on dissuading Israel from taking action - and distancing itself from an Israeli strike if persuasion fails.
The consequences of an Israeli attack would be wide-ranging and destabilising.
IRAN EXPELS NUCLEAR INSPECTORS, QUITS TREATY
Iran would expel International Atomic Energy Agency inspectors and quit the Nuclear Non-Proliferation Treaty, ending any possibility of a negotiated solution to the nuclear issue.
"There is not a country on Earth that is going to blame them for doing that, they are all going to blame Israel. Once Iran is out of the NPT, the sanctions are gone," Ken Pollack, director of the Saban Center for Middle East Policy at the Brookings Institution, told Reuters.
Leaving the NPT would disrupt the sanctions regime since its measures are predicated upon enforcing treaty compliance.
IRAN DECIDES TO GO FOR THE BOMB
Experts say a raid would only delay, not destroy Iran's programme. And once it had recovered, Iran would probably seek to develop nuclear weapons. Jon Alterman, director of the Middle East program at Washington's Center for International and Strategic Studies, told Reuters Iran would redouble "efforts to develop a deterrent so it never happens again."
OIL PRICES SOAR, JOLT FINANCIAL MARKETS
A strike on Iran and Iran's response, including attempts to close the Strait of Hormuz, which is vital for oil shipments, or an attack on Saudi oilfields, would lead to a sharp rise in oil prices that could seriously harm the U.S. economy, jeopardizing President Barack Obama's chances for re-election.
Saudi Arabia would be forced to use all its spare output capacity, a crucial safety cushion for oil markets.
But the most serious fears debated at oil trading desks include the possibility of Iran mining the straits, attacking ships as it did during the Iran-Iraq war, or challenging the legality of the passage of some vessels through its territorial waters.
In the event of a big stoppage the consuming nations' International Energy Agency would very likely release emergency government stocks to tame prices, as it did in June last year when Libyan output was lost.
Israel's Ayalon argues that a nuclear-armed Iran would pose a greater threat to the oil market as it could dictate prices. "It will be the end of the free flow of oil from the Gulf."
IRAN HITS BACK IN THE GULF, LEVANT, POSSIBLY ASIA
Tehran has warned several times it may seal off the Strait of Hormuz, choking the supply of Gulf crude and gas, if attacked or if sanctions mean it cannot export its oil.
But many experts say Iran's leaders will be looking for ways to harass enemies and cause disruption while falling short of triggering massive U.S.-led retaliation.
Possible Iranian actions could include harrying tanker traffic in the Gulf with fast attack boats, seizing uninhabited Gulf islands claimed by other states or grabbing hostages from passing civilian or military ships, stoking trouble in Sunni Muslim-ruled Arab states with restive Shi'ite Muslim communities and orchestrating attacks on U.S. forces in Afghanistan or elsewhere using militant "proxies" such as Hezbollah.
If the Iranian government interprets the strike as a fully-fledged attempt at regime change, it might adopt a more muscular response could include ballistic-missile salvos on civilian and military targets in the Gulf.
US GETS SUCKED IN, MAY ITSELF BECOME TARGET
Obama also would likely come under intense domestic pressure to back Israel's actions and come to Israel's defence if Iran succeeds in landing missile attacks on Israel's territory.
In a 2009 study for the Council on Foreign Relations, Middle East analyst Steve Simon, who is now Senior Director for the Middle East and North Africa at the U.S. National Security Council, says that the United States would probably become embroiled militarily in any Iranian retaliation against Israel or other countries in the region.
Experts say Israel alone does not have the firepower to kill off Iran's nuclear programme and any U.S. help in that effort would therefore be very welcome. A study by former senior British intelligence official said the "The US would be assumed complicit, and would become embroiled in defending Israel against a counter-attack. This would stretch the U.S. military."
STRAIN IN ISRAELI-WESTERN TIES
In November, the top U.S. military officer told Reuters he did not know whether Israel would alert the United States ahead of time if it decided to take military action against Iran.
General Martin Dempsey, chairman of the Joint Chiefs of Staff, also acknowledged differences in perspective between the United States and Israel over the best way to handle Iran.