The factor that the US would bring to a conflict with Iran is the ability to bomb its underground nuclear development centres.

The US is the only power to have the GBU-57 Massive Ordnance Penetrator. It weighs 18,600kg and can purportedly go through up to 100 metres of reinforced concrete. Its conventional explosive power is close to that of an atom bomb.

The US also has bombers built to carry heavyweight munitions like it, including the Northrop B-2 Spirit and the Boeing B-52. There have been reports backed up by satellite photos of six B-2s and four B-52s stationed at the Diego Garcia airbase in the Indian Ocean, 4,000km from Iran.

Washington can also provide a fleet of air-to-air refuelling tankers to assist Israel’s fighters, as well as the world’s biggest and most advanced air force. But given that the Israeli air force has bombed Iran’s surface-to-air missiles and radar defences, the number of targets is diminishing.

Already there are reports of the main refuellers – the Boeing KC-46A Pegasus and Boeing KC-135 Stratotankers – heading to Europe.

Explained: How US involvement could affect Israel-Iran war

‘Istar soak’

But another feature that America can provide is its military surveillance satellite system that delivers what is referred to as “Istar soak”.

While Israel has three spy satellites, the US has dozens that give it persistent intelligence, surveillance, target acquisition and reconnaissance (Istar) coverage that will allow it to track Iranian missile launches as well as senior commanders, government figures or nuclear scientists. This will enable “dynamic strikes” where weapons officers may see a target of opportunity and bring firepower to bear.

They could also help to track where Iran’s enriched uranium stockpiles might be, as well as centrifuges used in their manufacture.

Tomahawk attack

The US has about eight Arleigh Burke DDG-51 guided-missile destroyers in the region, most guarding the aircraft carrier the USS Carl Vinson. The US Navy warships each carry between eight and 10 Tomahawk cruise missiles.

If it was decided that more Iranian airbases had to be destroyed, Tomahawks may be sent in.

The USS Nimitz carrier, with its 90 aircraft, is steaming to the area from the Pacific and will arrive in days.

America can also use its special forces for strike missions, its intelligence and surveillance aircraft and its military satellites that would vastly increase the information-gathering mission.

In addition, there is the possibility that America will send more of its own air defences to Israel and possibly the Gulf. It has a Thaad high-altitude interceptor battery near Tel Aviv and Patriot missile batteries in the region.

The Central Command based in Doha, Qatar, is staffed by high-ranking US and western officers who would be able to co-ordinate all of the offensive operations.

War spreads?

While others have remained apart from the conflict, countries such as Britain could be drawn in if, for example, Tehran was to fire one of its Fatah-1 hypersonic missiles at RAF Akrotiri in Cyprus.

This would cause a political problem and destabilise the western alliance as the UK would have to choose whether to join Israel in the war, despite being at diplomatic loggerheads over the country’s conduct in Gaza.

Similarly, France has a number of military bases in the region that could be attacked and, alongside Britain, warships in the area.