In the face of such a multi-pronged assault, the regime might even be wondering whether they, or rather their nuclear sites - seen as by far the biggest threat to Israel - could be next.

Israel-Hezbollah latest: Hezbollah warns Israel 'we are ready' for ground invasion

Unspoken rules that have in the past deterred a direct Israeli attack on Iran have been ripped up following the killing of the leader of Hezbollah in Lebanon, the dismantling of much of Hamas in Gaza, and large airstrikes against Houthi targets in Yemen.

Israeli troops are now preparing for a possible ground incursion into Lebanon as they seek to inflict lasting damage on Hezbollah, the region's largest paramilitary force and Iran's most prized insurance policy.

The speed and intensity of events make it impossible to predict what will happen next. A priority for Iran though will be to try to restore deterrence - yet it might already be too late.

Israel's actions appear designed to achieve a fundamental shift in the balance of power in the region, where Tehran's "axis of resistance" - armed groups, funded, equipped and trained by Iran and aligned with its interests - has been a hugely powerful force.

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The Iranian president on Sunday night denounced the Israeli attacks against Hezbollah, Hamas and the Houthis and vowed to respond - though without indicating how.

Masoud Pezeshkian just said: "We cannot accept such actions and they will not be left unanswered."

In considering its next move, Tehran will have drawn lessons from its response in April to an Israeli strike against an Iranian consulate building in the Syrian capital, Damascus.

At that time, Iran launched hundreds of missiles and drones towards Israel - at the higher end of expectations - but the majority of the barrage was defeated by Israeli air defences supported by the US, UK and other allies.

Israel was encouraged to "take the win" and not strike back too hard - advice that it heeded in the immediate aftermath and a descent into uncontrolled confrontation was avoided.

Yet three months later, the Israelis escalated tensions once again by killing Ismail Haniyeh, a top Hamas leader, while he was in Tehran.

That provocation triggered new expectations of retaliation which have yet to materialise - an Iranian silence that may well have emboldened Israel to cross further red lines, right up to the taking out of Hassan Nasrallah, the leader of Hezbollah, last Friday.

Matthew Savill, director of military sciences at the Royal United Services Institute, said Iran has a reputation for calibrating its operations to avoid escalation to direct war with Israel.

"But it seems to have badly miscalculated how much risk the Israelis - and Netanyahu in particular - are willing to take after 7 October, given the shock that produced in Israel and the level of threat it created," he said.

"The so-called axis of resistance is meant to be a deterrent to attacks on Iran. But Iran probably won't risk direct war on behalf of its partners.

"So while a ground incursion into Lebanon would be difficult, the Israelis might pay the cost if they can deal a generational blow to Hezbollah. And many will argue, given the loss of its leadership and its communications problems, if not now, then when?"

If such a blow is possible, might Israel then have the capacity, capability and confidence to turn its sights directly on its biggest enemy, Iran?

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Benjamin Netanyahu, the prime minister, has relentlessly warned of the danger posed to his country by Tehran, which he believes is intent on acquiring nuclear weapons - a move that Israel would not tolerate.

The Iranian government denies such a plan, saying its nuclear facilities are purely for civilian use.

In the past, calculations about Iran by Israel and its closest ally, the US, were based on an understanding that a direct attack would risk Hezbollah - in Iran's defence - unleashing deadly volleys of missiles in Israel's direction.

That threat still exists, but the killing of Hassan Nasrallah and his top lieutenants in Israeli airstrikes as well as the destruction of weapons sites across Lebanon have dealt punishing blows to the group, which will impact its ability to fight back at least in the short term.

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Tehran's Islamic Revolutionary Guard Corps (IRGC) of course has a potent arsenal of missiles of its own, bolstered with new technology and capabilities from its partner Russia.

It means any direct confrontation would be catastrophic for the region - something that Iran and Israel will both be all too aware of as they decide responses and counter-responses.

The United States appears increasingly worried about what comes next if calls for a ceasefire between Israel and Hezbollah as well as Israel and Hamas continue to go unheeded.

The Middle East has been on the brink of all-out war for almost a year but this is by far the most dangerous moment.