The Department of Defense raised the security posture at Mildenhall and the neighbouring RAF Lakenheath to Force Protection Condition Charlie, a level that indicates a terrorist action is likely. A spokesperson for US European Command declined to detail specific measures. The spokesperson said both bases remain vigilant, in close partnership with host nation allies, to keep US service members and civilians safe. A US Air Force official said aircraft rotations happen for various reasons. US officials are still discussing the precise nature of the Iranian threat, including whether it is imminent or more general. Secretary of State Marco Rubio said, "We've addressed it by moving people away from the threat."

The UK Ministry of Defence declined to comment on the movements. A spokesperson said the UK does not routinely discuss other nations' activities and that aircraft are regularly moved according to operational requirements. Two UK officials confirmed the operation.

Mildenhall is the second British base affected. US B-1 bombers were earlier withdrawn from RAF Fairford in southwest England after a security incident that British authorities have tied to Iran. Accounts differ on how many men were arrested, but two were later charged on suspicion of assisting Iranian intelligence, and two British nationals appeared in court in October. Prime Minister Andy Burnham has said the Fairford activity was likely coordinated in Iran. Iran's ambassador in London denied any involvement. The Islamic Revolutionary Guard Corps declared in July that Fairford and other bases used for strikes against Iran are legitimate targets.

Iran and the United States have been at war since February. On Saturday, Iranian Army spokesperson Mohammad Akraminia said Iranian forces are stronger than they were at the start of the war and would strike US targets if attacked. Mohsen Rezaei, secretary of Iran's Supreme National Security Council, accused Washington of spreading fake news and urged it to "surrender" to Tehran's demands. Iran has also sent the US a nuclear proposal through Qatar that carries seven conditions. President Donald Trump met Qatari Prime Minister Sheikh Mohammed bin Abdulrahman bin Jassim Al Thani on Monday.

US intelligence has detected a substantial increase in Iranian attempts to target American diplomats, with some plots at advanced stages.

In Britain, Conservative leader Kemi Badenoch called the latest withdrawal "very concerning" and said the government had "absolutely nothing to say about it." MI5 director general Ken McCallum has said threats to the UK from foreign adversaries have quadrupled in the last decade, citing increased Iran-sponsored aggression against Jewish sites and military bases.