Leo IV is consecrated pope. During the reign of his predecessor, Pope Sergius II, Muslims had invaded Italy and even sacked Ostia, so Leo IV is immediately occupied with dealing with this crisis. Everyone interprets the military success of the Muslim raiders as a sign of God's displeasure with Christian institutions.
Pope Leo IV organizes the efforts to protect Rome from destruction, including the construction of a 40-foot high wall surrounding St. Peter's and much of the Vatican, as well as other fortifications and defenses for the city generally. The fortifications immediately around the Vatican came to be known as the "Leonine City."
Pope Leo IV also organizes a military alliance among Greek and Italian cities so he can go on the offensive and drive the Muslim raiders out of Italy.