Five men were arrested on suspicion of explosive and terrorism offences after armed police intercepted three vans travelling towards the military airfield Royal Air Force base Fairford in the west of England, in the early hours of Sunday.
The group, detained in the nearby village of Whelford, Gloucestershire, were "looking to do big damage", US President Donald Trump said later.
A local farmer returning from a party called police after spotting the vehicles "blocking the road" before seeing a "large group of hooded and 'balaclava'd' men".
The woman told the Press Association "they would have seen the bright headlights" and "they probably thought we were the police (and) panicked".
"It would appear that by doing so I just derailed whatever they were planning," she said.
"A plucky amateur shouldn't be spotting an unusual bit of activity at 1am when the air base is on high alert," she said.
"I think it's absolutely astonishing."
The British government had authorised the US to use bombers based at Fairford to strike Iranian missile sites that were attacking ships in the Strait of Hormuz.
Britain said in July its armed forces were ready to protect the country from any attack after Iran's Revolutionary Guards warned it would not allow US bombers to fly from Fairford, saying any base used to launch attacks would be a legitimate target.
Police arrested the men under the Explosives Act and they were later further arrested under suspicion of preparation of a terrorist act.
Suspects detained under the Terrorism Act 2000 can be held for up to 14 days without charge, compared to the 24-hour limit for standard arrests without an extension application.
Aerial footage showed a bomb disposal robot spending hours inspecting three white vans, two of which had open rear doors. Large black barrels appeared to be scattered around.
A 400-metre cordon was erected and 85 evacuated households were unable to return home overnight.
The Army's ordnance disposal unit was still examining the vehicles in the cordon by the evening, Temporary Chief Constable Maggie Blyth told a press conference.
Agencies "believe the matter to be contained", she said.
Prime Minister Andy Burnham said the incident was "deeply concerning" and the government would "always take every step necessary" to protect people.
Investigators were now working to analyse any devices seized from those arrested to establish details of any plot and any potential motivation.
The arrests come amid heightened concerns around the threat from hostile states to the UK, with Russia and Iran among those that have used proxies to commit crimes on their behalf on British soil.
In March, officials said the number of national security cases involving activity by hostile states including Iran increased 50 per cent in the six months to December 2025.
RAF Lakenheath in Suffolk confirmed it was on Charlie alert, which is the second-highest threat level that can be issued by the US Department of Defence.
The confirmation to PA came amid reports that RAF Fairford has been raised to the highest threat level, Delta.
A spokesperson for the USAF said its personnel based at RAF Fairford "remain vigilant", but "for operational security reasons we will not discuss specific force protection measures".
Trump praised the UK response and suggested authorities already had the suspects under investigation, despite others' claims to the contrary.
"They were looking to do big damage to our fort, and working with the British worked out great," he told reporters in the US.
"We had them under view for a long time, and we got them."
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