Pope Leo IV was the bishop of Rome and ruler of the Papal States from 10 April 847 to his death. He is remembered for repairing Roman churches that had been damaged during the Arab raid against Rome, and for building the Leonine Wall around Vatican Hill to protect the city. Pope Leo organized a league of Italian cities who fought and won the sea Battle of Ostia against the Saracens.

The Arab raid against Rome took place in 846. Muslim raiders plundered the outskirts of the city of Rome, sacking the basilicas of Old St Peter's and St Paul's-Outside-the-Walls, but were prevented from entering the city itself by the Aurelian Walls.