The U.S. on Friday began retaliatory strikes in the Middle East, likely against Iran-backed militants for a drone strike on an American base in Jordan last Sunday that killed three U.S. service members.

Dozens of other American troops were wounded in the drone attack on the Tower 22 base near Jordan’s border with Iraq and Syria. The U.S. says Iran is responsible for funding and arming the militants while Iran has denied involvement.

President Joe Biden had quickly warned that America would respond, escalating involvement in the Middle East after months of trying to contain tensions from boiling over into a broader war in the region.