Short answers, pulled from the story.
The Battle of Monte Cassino was a series of four Allied assaults against German positions in Italy during World War II, fought between the 17th of January and the 18th of May 1944. The objective was to break through the German Gustav Line and open the road to Rome along Highway 6 through the Liri valley.
Allied commanders believed the historic Benedictine abbey was being used by German forces as an artillery observation post, though evidence was disputed. On the 15th of February 1944, 142 Boeing B-17 Flying Fortresses and dozens of medium bombers dropped 1,150 tonnes of high explosives on the abbey. Subsequent investigations established that the only people killed inside were 230 Italian civilians; the abbey was actually unoccupied by German troops at the time.
Total Allied casualties across the four Cassino battles, the Anzio campaign, and the capture of Rome exceeded 105,000, while Axis casualties were at least 80,000. The town of Cassino was completely razed, and 2,026 of its prewar population of 20,000 were killed during the raids and battle.
A patrol of the Polish 12th Podolian Cavalry Regiment raised the first flag over the ruins of the Monte Cassino monastery on the 18th of May 1944 at 10:20 in the morning. The Polish flag was followed by the British flag shortly after. All members of the Polish units who fought at Cassino were awarded the Monte Cassino Commemorative Cross.
Captain Maximilian Becker and Lieutenant Colonel Julius Schlegel of the Hermann Goering Panzer Division organised the evacuation of the abbey's contents to Rome in the autumn of 1943. In more than 100 truckloads completed by early November 1943, they removed 800 papal documents, over 80,000 volumes, 500 incunabula, 200 parchment manuscripts, 100,000 prints, and paintings including works by Titian, El Greco, and Goya.
The Polish II Corps, commanded by Lieutenant General Wladyslaw Anders, carried out two major assaults on Monte Cassino. The first assault on 11-the 12th of May 1944 cost 281 officers and 3,503 other ranks. The second attack on the 17th of May led to the German withdrawal, allowing a Polish patrol to raise their flag over the ruins on the 18th of May 1944.
American writer Walter M. Miller Jr. served as part of a bomber crew that participated in the destruction of the Monte Cassino monastery. Miller stated the experience directly influenced him and led him to write A Canticle for Leibowitz a decade later, a science fiction novel about monks preserving the remnants of human knowledge after a nuclear war.