The first battle started on January 4, 1944 and the monastery atop the hill was destroyed by Allied bombing on February 15. Allies aircraft heavily bombed the ruins of the monastery and staged an assault on March 15.
During three failed attempts to take the heavily-guarded monastery of Monte Cassino (January 17-25, February 15-February 18, March 15-March 25), the forces of USA, UK, India and New Zealand lost approximately 54.000 men yet did not manage to seize the city or the castle overlooking the Rapido river valley.
So called Fourth Battle of Monte Cassino was fought by the 2nd Polish Corps under General Wladyslaw Anders (May 11-May 19). The first assault (May 11-May 12) brought heavy losses but also allowed the British 8th Army under General Sir Oliver Leese to break through German lines in the Liri river valley below the monastery.
The second assault (May 17-May 19), carried out at immense cost by the Polish troops with the aid of a force of Morroccan hill tribesmen used to mountain warfare, pushed the German 1st Parachute Division out of its positions on the hills surrounding the monastery and almost surrounded them. In the early morning of May 18 a recconnaissence group of Polish 12th Podolian Ulans Regiment seized the ruins.
The capture of Cassino allowed the British and American divisions to begin the advance on Rome, which fell on June 4 1944 just days before the Normandy invasion.