In early 1944 the Germans were holding what was called the Winter Line. the Germans were holding Rapido-Gari, Liri and Garigliano valleys. This was called the Gustav LIne. In the middle of it was the Abbey of Monte Cassino. It dominated the entrances to the Liri and Rapido Valleys. The Germans had said they would not put troops in the castle, however Allied forces were convinced that the German were using the abbey to spot Allied troops. It was decided to the attack the Abbey. On February 14th American bombers dropped 1,400 tons of bombs on the Abbey turning much of it into ruins. Now that it was ruins the German decided it would make an excellent defensive position and sent thousands of troops to man it.
The capture of the Abbey soon became the pivotal point in the Gustav Line. It was considered the key to breaking through the line and capturing Rome. The first assault began on January 17th, it was hoped that the assault coming together with the Anzio landing would succeed in convincing the Germans to pull out to lines North of Rome. After three days of fighting it was clear that this was not going to happen and the assault came to an end.
The second attempt took place on February 17th. The key to that assault was suppose to be the use of Gurkahas who had special experience fighting in the mountains. That also failed.
The third attempt to capture the Abbey started on March15th. It was accompanied by massive bombing and artillery barrage. The New Zealand Division took the lead. Despite fierce fight the assault was no more successful than the pervious ones.
The Third and final assault began on May 11th. It too began with a massive bombardment. this time the attack came all along the line. The US Gift and the French Expeditionary forces achieved most of their objectives quickly. the British XIII corp were able to bring tanks up to defend any German attempt at a counterattack. Meanwhile elements of the a Polish army Division fought in the hills above the abbey. They were met with fierce resistance by the Germans. By May 18th however the Poles had achieved the upper hand and were breaking through German lines. The German finally decided their position was untenable and those able began to withdraw to the North.
The cost of capturing Monte Cassino was very high. The allies had 50,000 casualties while the Germans had 20,000.