The head of the UK armed forces told Sky News over the summer that he does not believe any of the UK's opponents - including Tehran, Moscow and Beijing - would be able to defeat the scale of attack that Iran first launched towards Israel in April.

"I think the US leadership and the proficiency that we have with our allies is at a level above our potential foes," Admiral Sir Tony Radakin said in an interview.

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That attack in April involved more than 100 ballistic missiles, nearly 200 drones and tens of cruise missiles - which were largely defeated.

Israel and the United States played by far the biggest role in blasting them out of the sky but the UK and a number of other allies also offered support.

This time around, the Iranian military fired almost twice as many ballistic missiles - the hardest type of weapon to intercept because of the speed they can travel at.

But Israel said most of the projectiles were again intercepted in an operation once more led by Israeli and US forces.

While some of the projectiles did penetrate the defences, the damage was seemingly limited.

It is an outcome that the US in particular will be hoping might limit the size of the Israeli retaliation.

This is what happened in April when the Netanyahu government was urged to "take the win".

But Israel looks to be in no mood to compromise as its forces exploit the momentum from more than two weeks of punishing attacks against Hezbollah, the Iranian-backed paramilitary force in Lebanon, including the killing of its leader, which part-prompted the Iranian missile strike.

Should the Israelis - as anticipated - launch an air attack on Iran, the damage that the Iranian regime would incur would likely be a lot bigger than what Israel absorbed unless the regime too is able to muster up some kind of coalition with its allies, principally Russia.

That seems highly unlikely given Russian forces will want to preserve their air defences to protect their own skies from increasing strikes by Ukraine - although the Russian government has been supplying Iran with increasingly sophisticated air defence systems, reportedly including the S-400, which is a step up from the Russian S-300 that Iran already operates.

The frailties in Iran's own ability to defend its skies were exposed in January 2020 when a Revolutionary Guards air defence unit mistakenly shot down a Ukrainian passenger aircraft, killing all 176 people on board.

The error happened during a previous time of regional tensions when Iran was bracing for possible US retaliation to an Iranian attack against US forces in Iraq.

Air defence like a 'game of 3D chequers'

The task of air defence is high-pressure and hugely complex.

It relies on a layered set of defences that complement each other, and need to be closely coordinated, while involving rapid decision-making and action.

"Early radar and satellite warning is key," said a former senior Royal Air Force officer.

Images and movement picked up by satellite and radar are then used to identify possible threats, track them and work out if they are friendly, hostile or still unknown.

Next, commanders must prioritise the hostile threats, before instructing the appropriate air defence team to open fire.

Different threats require varying types of air defence systems, from short-range ground-based weapons to longer-range ones, with aircraft also able to play a role.

"So, a game of 3D chequers - putting in appropriate blocking pieces," the former RAF officer said.

Ballistic missiles are typically countered by large ground or sea-based air defence platforms that can even destroy the missile while it is out of the Earth's atmosphere - which is almost certainly what will have happened to chunks of the incoming Iranian arsenal.

I watched some of the intercepts high up in the sky above northern Israel - they could well have been exoatmospheric (outside the Earth's atmosphere).

Adding to the complexity of the challenge of shooting down incoming projectiles is Iran's geography.

Geographically, it is a much bigger country than Israel, meaning finite air defence systems will only be able to guard the most important people and sites - in particular nuclear facilities.

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On the flip side, the distance between Iran and Israel creates difficulties for the attacking force.

Israel has a far more sophisticated air force than the Iranians so may well seek to strike Iran with cruise missiles and other bombs launched from jets rather than ballistic missiles.

But they will need to fly more than 1,000 miles to hit their targets, laden with bombs and refuelling multiple times in the air - a mission that will be impossible to hide in advance.

Yet this is a mission Israel will have trained for.

It might well be just a matter of time until its aircraft fly the sortie for real.