The U.S. military’s decision to pull its B-1 bombers from a base in the United Kingdom is representative of the growing threat the Air Force has to contend with when it comes to defending its aircraft — not while they’re in the sky, but when they’re on the ground.

President Donald Trump said the dozen bombers brought back stateside from Royal Air Force Fairford were due to “some threats,” and he warned that Iran would “suffer greatly” if they had smuggled attack drones into the U.K. for an attack targeting the base. It came days after U.K. authorities thwarted an apparent terrorist plot tied to Iran, which has resulted in the arrests of a half dozen individuals.