The recent U.S. deployment of B-2 bombers, the only planes able to launch the most powerful bunker-busting bombs, to within range of Iran is a potent signal to Tehran of what could happen to its nuclear programme if no deal is reached to rein it in.
But military and nuclear experts say that even with such massive firepower, U.S.-Israeli military action would probably only temporarily set back a programme the West fears is already aimed at producing atom bombs one day, although Iran denies it.
Worse, an attack could prompt Iran to kick out United Nations nuclear inspectors, drive the already partly buried programme fully underground and race towards becoming a nuclear-armed state, both ensuring and hastening that feared outcome.
"Ultimately, short of regime change or occupation, it's pretty difficult to see how military strikes could destroy Iran's path to a nuclear weapon," said Justin Bronk, senior research fellow for airpower and technology at the Royal United Services Institute, a British defence think-tank.
"It would be a case of essentially trying to reimpose a measure of military deterrence, impose cost and push back breakout times back to where we were a few years ago."
Breakout time refers to how long it would take to produce enough fissile material for a nuclear bomb, currently days or weeks for Iran. Actually making a bomb, should Iran decide to, would take longer.
The landmark 2015 nuclear deal between Iran and major powers placed tough restrictions on Iran's nuclear activities that increased its breakout time to at least a year. After President Donald Trump pulled the United States out of the deal in 2018 it then unravelled, and Iran pushed far beyond its limits.
Now Trump wants to negotiate new nuclear restrictions in talks that began last weekend. He also said two weeks ago: "If they don't make a deal, there will be bombing."
Israel has made similar threats. Its Defence Minister Israel Katz said after taking office in November: "Iran is more exposed than ever to strikes on its nuclear facilities. We have the opportunity to achieve our most important goal – to thwart and eliminate the existential threat to the State of Israel."
BIG, RISKY
Iran's nuclear programme is spread over many sites, and an attack would likely have to hit most or all of them. Even the International Atomic Energy Agency, the U.N. nuclear watchdog, does not know where Iran keeps some vital equipment, like parts for centrifuges, the machines that enrich uranium.
Israel could take out most of those sites by itself, military experts say, but it would be a risky operation involving repeated attacks and would have to deal with Russian-supplied anti-aircraft systems - although it managed to do so in far more limited strikes on Iran last year.
Uranium enrichment is at the heart of Iran's nuclear programme, and its two biggest enrichment sites are the Fuel Enrichment Plant at Natanz, located about three floors underground, apparently to protect it from bombardment, and Fordow, dug far deeper into a mountain.
The United States would be far better equipped to take out those hard targets with its most powerful bunker buster, the 30,000-pound (14,000 kg) Massive Ordnance Penetrator, which only B-2 bombers like those recently moved to Diego Garcia in the Indian Ocean can currently fire and which Israel does not have.
"(Israel) don't have enough 5,000 pounders," to take out Fordow and Natanz, said retired U.S. Air Force General Charles Wald, who now works for the Jewish Institute for the National Security of America, which promotes close defence ties between the United States and Israel.
He was referring to the biggest bunker-buster in Israel's arsenal. With the U.S., an attack would be quicker and more likely to succeed, though Wald estimated it would still take days.
THEN WHAT?
"A strike by the United States could probably cause more damage than an Israeli strike, but in either case you're talking about buying time and there's a real risk that it drives Iran toward rather than away from a bomb," said Eric Brewer of the Nuclear Threat Initiative, a former U.S. intelligence analyst.
"A strike can disrupt and delay the program, but it can't destroy it."
While sites can be eliminated, Iran's now advanced knowledge of uranium enrichment cannot. Preventing rebuilding would be a constant, extremely difficult task, analysts and officials said.
"What happens the day after? Iran responds to attacks on its nuclear program by hardening its facilities and expanding its program," said Kelsey Davenport of the Arms Control Association.
Having already undone additional IAEA oversight introduced by the 2015 deal, many analysts see a risk that, if attacked, Iran would kick out IAEA inspectors acting as the world's eyes at sites like Natanz and Fordow.
"The continuation of external threats and Iran being in a state of #military_attack may lead to deterrent measures, including #expulsion_of_inspectors from IAEA and cessation of cooperation," Ali Shamkhani, long Iran's top security official and now an adviser to Supreme Leader Ali Khamenei, said on X last week.
The only other country to do that is North Korea, which then tested its first nuclear bomb.
"If you bomb Iran, Iran is going to almost certainly in my judgement chuck out international inspectors, make a dash for the bomb," said James Acton of the Carnegie Endowment for International Peace.
