A sudden security escalation at two major US military bases in Britain has triggered questions over a possible threat to American forces in Europe. US authorities have raised the alert level at RAF Mildenhall and RAF Lakenheath to the second-highest security category, while several military aircraft have unexpectedly moved out of Mildenhall.According to a report by The War Zone (TWZ), the US military confirmed enhanced force protection measures at the two installations on November 9 but did not disclose what prompted the decision.A subsequent Wall Street Journal report, said both bases had been placed under Force Protection Condition Charlie (FPCON Charlie), a level activated when intelligence indicates terrorist activity is likely.The development comes amid renewed concerns over Iran-linked threats against American military installations overseas, following a security incident near another US-operated airbase in Britain earlier this year.In September 2026, British authorities arrested five men near RAF Fairford after three vans were found in the vicinity of the military installation. Although no explosives were discovered, the investigation subsequently led to two further arrests. One suspect was charged with assisting a foreign intelligence service believed by British police to be Iran.The incident raised concerns about possible Iranian intelligence operations targeting US military assets in Britain. Tehran has denied involvement in the alleged plot.RAF Fairford had previously been used by American B-1B Lancer strategic bombers during military operations against Iran. The US subsequently withdrew the bombers from the installation amid concerns about potential threats.Unexpected aircraft movements from MildenhallThe latest security escalation coincided with unusual movements of American military aircraft from RAF Mildenhall.Flight-tracking observers reported that several KC-135 Stratotanker aerial refuelling aircraft were moved to other European locations, including Germany, Spain and Scotland. CV-22B Osprey special operations aircraft were also relocated from Mildenhall to RAF Lakenheath.According to subsequent reporting by Sky News, nine KC-135 tankers and three Ospreys were involved in the movements.The US Air Force said aircraft routinely move between installations for operational requirements and training. However, it did not establish whether these particular relocations were directly connected to the heightened security alert.RAF Mildenhall is a major American military hub in Britain. It hosts KC-135 Stratotankers, which extend the operational range of US fighter jets and bombers through aerial refuelling. The installation also houses RC-135 Rivet Joint reconnaissance aircraft, which collect intelligence and monitor military activity, alongside special operations aircraft.RAF Lakenheath, meanwhile, hosts F-15E Strike Eagle fighter-bombers and F-35A Lightning II stealth fighters. It is one of the most important US fighter installations in Europe and provides Washington with significant combat air capabilities in the region.Drone threat raises fresh concerns over US basesThe latest security alert has also drawn attention to the possibility of unmanned aerial systems being used to surveil or threaten American military installations in Britain. The concern is not without precedent.In November 2024, unidentified drones were repeatedly spotted flying near and over four UK airbases hosting US forces, including RAF Lakenheath, RAF Mildenhall, RAF Feltwell and RAF Fairford.The issue remains relevant because small drones can potentially be used to monitor aircraft movements, map security arrangements and gather intelligence around sensitive military infrastructure. Depending on their capabilities and payloads, they can also pose risks to flight operations and personnel.The possibility of such activity has resurfaced after RAF Lakenheath issued a public warning on November 9, urging residents to report suspicious activity, including unauthorised drone operations, around the installation.