Isernia was known as
Samnite Aesernia until it fell under
Roman rule in 263 BC. The territory was later given to
Alczeco for his efforts against the Byzantines with his
Bulgarian military, by the
Duchy of Benevento; most of this territory became Molise County. Throughout the 9th century the area was repeatedly sacked during Muslim invasions and suffered an earthquake in 847; these destroyed the cities of Isernia and
Venafro. In Molise County, Benedictines formed the
San Vincenzo al Volturno monastery in the province of Isernia after Rome fell in 476. When the area was invaded by the Muslims, 500 to 900 monks were decapitated by them and many were taken as slaves; despite this, the monastery continued to function and reached its peak size during the eleventh and twelfth centuries.
Pope Celestine V was born near Isernia and the province's feast day is on 19 May, the date Celestine V was canonized. An earthquake in 1805 destroyed Isernia's cathedral, which was rebuilt atop the ruins of the destroyed structure in 1837. ==Geography==