By the end of December 1943 the advance of the Allied forces in Italy was hampered by strong German defences on the Gustav or Winter Line. The area around the town of Cassino with its heavily fortified mountain defences and difficult river crossings was the key position on the Gustav Line.
Four times the Allies tried to break through the Monte Cassino stronghold. The first battle took place between 17 January and 11 February 1944 with heavy losses and no success for the Allies. To relieve pressure on Anzio Beachhead where the allied were pinned down by heavy German resistance, a second battle was launched between 16 and 18 February.
On 15 February the famous historic abbey of Monte Cassino was destroyed by American bombers. Allied command was convinced that the ancient abbey was a German observation post. Ironically German troops occupied the ruins only after the air raid. The third battle was conducted between 15 and 23 March, again without success. The fourth battle began on 11 May. Finally the Germans retreated from the Gustav Line on 25 May 1944.
After five months of stalemate on the Gustave Line the road to Rome lay open. The costs were high. It is estimated that the Allies (Australia, Canada, Free France employing also Moroccans, Kingdom of Italy, India, New Zealand, Poland, South Africa, United Kingdom, and the U.S.) suffered about 55,000 casualties, Germany and the Italian Social Republic about 20,000.