News of a massing of Saracen ships off Sardinia reaches Rome early in 849.

When the Muslims approach Portus, Pope Leo summons the Repubbliche Marinare (or mariner cities of Italy)—Naples, Gaeta and Amalfi—to form a league.

The command of the unified fleet is given to Cesarius, son of Duke Sergius I of Naples.

The Christian armada assembles off recently refortified Ostia, and Pope Leo IV comes out to bless it and offer a mass to the troops.

The subsequent Battle of Ostia is one of the most famous in history of the papacy of the Middle Ages and is celebrated in a famous fresco by Raphael and his pupils in his Rooms of the Vatican Palace in the Vatican City.

After the pirate ships appear, battle is joined with the Neapolitan galleys in the lead.

Midway through the engagement, a storm divides the Muslims and the Christian ships manage to return to port.

The Saracens, however, are scattered far and wide, with many ships lost and others sent ashore.

When the storm dies down, the remnants of the Arab fleet are easily picked off, with many prisoners taken.

In the aftermath of the battle, much booty washes ashore and is pillaged by the locals, per ius naufragii.

The prisoners taken in battle are sent to work in chain gangs building the Leonine Wall which is to encompass the Vatican Hill.

Rome will never again be threatened by a Muslim army.