In 529 AD St Benedict of Nursia came to settle on the site, founded a monastery and there wrote the rule of St Benedict, which became the basis of Western monasticism. There the Benedictine monks established what may have been Europe's first hospital. But the monastery did not last long after Benedict's death,for it was sacked by invading Lombards, a German tribe, and abandoned for a century. .
The new abbey on the site lasted until the ninth century when it was burned down by invading Saracens, members of an expansionist Muslim tribe who managed to hold Sicily and parts of southern Italy.The abbey was then rebuilt under Abbot Desiderius, and despite damage in an earthquake in 1349 managed to survive until the twentieth century. During this time it was pillaged by Napoleon's troops,but the structure of the building was left intact. In the time of Italian unification when the monasteries were suppressed it became the property of the Italian state, was classed as a national monument but the monks left in charge. [Source, Catholic Herald, Allan Mallinson, May 17th 2019.]
The German occupation was extremely disciplined and civilised. Field Martial Kesselring had left Monte Cassino out of the Gustav line fortifications as it was a cultural treasure, and through the Red Cross informed the allies of his decision, but there were German positions on the hill around the monastery. In November 1943 Lieutenant-Colonel von Schlegel, a Catholic, and medical officer Maximilian Becker, a Protestant, foresaw danger and with the consent of General Von Senger made arrangements to move cultural artefacts, many of which belonged to the monastery and others to a museum. These cultural items were stored at the Vatican. Von Senger personally provided a German army staff car to take away the abbot, who had remained to supervise the removal, and took him to his chosen place of safety, another monastery in a safer area.
The Germans eventually,fearing that they were in danger of being surrounded, retreated across the hills to the Hitler Line, leaving mainly troops too badly wounded to move and a skeleton unit to cover the retreat. On the early morning of the 18th May 1944 a Polish patrol took the monastery with light casualties, and hoisted the Polish flag, the White Eagle of Poland. The monastery remained ruined until after the war.
The dead and wounded numbered 55000 Allied troops and 20000 Germans. The battle was a costly victory.