As so often with early Christian institutions, the monastery was constructed on an older pagan site, a temple of Apollo that crowned the hill, enclosed by a fortifying wall above the small town of Cassino, still largely pagan and recently devastated by the Goths. Benedict's first act was to smash the sculpture of Apollo and destroy the altar. He rededicated the site to John the Baptist. Once established there, Benedict never left. At Monte Cassino he wrote the Benedictine Rule that became the founding principle for western monasticism. There at Monte Cassino he received a visit from Totila, king of the Ostrogoths, in 580 (the only secure historical date for Benedict), and there he died.
Monte Cassino became a model for future developments. Unfortunately its protected site has always made it an object of strategic importance. It was sacked or destroyed a number of times. In 584 the Lombards sacked the Abbey, and the surviving monks fled to Rome, where they remained for more than a century. During this time the body of St Benedict was transferred to Fleury, the modern Saint-Benoit-sur-Loire near Orleans, France. A flourishing period of Monte Cassino followed its re-establishment in 718, when among the monks were Carloman, the son of Charles Martel, Rachis, brother of the great Lombard Duke Astolf, and Paul the Deacon, the historian of the Lombards. In 883 Saracens sacked and then burned it down.
It was rebuilt and reached the apex of its fame in the 11th century under the abbot Desiderius (abbot 1058 - 1087), who later became Pope Victor III, and abbot Oderius. The number of monks rose to over two hundred, and the library, the manuscripts produced in the scriptorium and the school of manuscript illuminators became famous throughout the West. The buildings of the monastery were reconstructed on a scale of great magnificence, artists being brought from Amalfi, Lombardy, and even Constantinople to supervise the various works. The abbey church, rebuilt and decorated with the utmost splendor, was consecrated in 1071 by Pope Alexander II. A detailed account of the abbey at this date exists in the Chronica monasterii Cassinensis of Leo of Ostia.
An earthquake damageded the Abbey in 1349, and although the site was rebuilt it marked the beginning of a long period of decline. In 1321 pope John XXII made the church of Monte Cassino a cathedral, and the carefully preserved independence of the monastery from episcopal interference was at an end. In 1505 the monastery was joined with that of St. Justina of Padua. The site was sacked by Napoleon's troops in 1799 and from the dissolution of the Italian monasteries in 1866, Monte Cassino became a national monument. There was a final destruction on February 15, 1944 when during the four battles of Monte Cassino (January - May 1944), the entire building was pulverized in a series of heavy air-raids. The Abbey was rebuilt after the war, financed by the Italian State. Pope Paul VI reconsecrated it in 1964.
The archives, besides a vast number of documents relating to the history of the abbey, contained some 1400 irreplaceable manuscript codices, chiefly patristic and historical. By great foresight, these were all transferred to the Vatican at the beginning of the war.