VIENNA - 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...