[Sidenote: Pippin gives the Exarchate to the Pope.] Their keys were brought to Rome and laid in triumph on the sepulchre of St. Peter. Thus did the Pope become an important secular prince, by taking over the old Byzantine dominions in central Italy. It would seem that the theory by which he justified this usurpation was that the guard of the possessions of the ‘Roman Republic’ in Italy was incumbent on the emperor, but that Constantine Copronymus being an obstinate heretic his rights fell into abeyance. The Pope then stepped forward as the representative of the ‘Roman Republic’ in default of a Caesar, and claimed possession of all that the Lombards had lately usurped. Apparently he considered himself as ‘Patrician’ in the Exarchate, but as a Patrician owing no duty or obedience to a heterodox emperor. King Aistulf died in the next year, killed by a fall from his horse, and the affairs of Italy troubled Pippin no more, Desiderius, duke of Istria, the new Lombard king, being occupied with strengthening himself against an attempt of the ex-king Ratchis to leave his cloister and resume the crown. The rest of Pippin’s reign was mainly devoted to the completion of the Frankish dominion in southern Gaul. Soon after his proclamation as king his officers had recovered for him all the Saracen towns in Septimania north of Narbonne. In 759 Pippin marched in person to lay siege to that city, the last bulwark of Islam beyond the Pyrenees. [Sidenote: Pippin takes Narbonne, 759.] The Christian inhabitants of the place rose at his approach, slew the Arab garrison, and opened their gates to the Frank. No help came from Spain, where civil war was—as usual—raging, and the boundaries of the realm of Pippin were advanced to the Pyrenees.