The Battle of Monte Cassino was actually a series of Allied offensives in Italy against the German Winter Line, which had in its center the monastery that would give the name of the famous battle.
The Winter Line was in fact composed of several defensive lines, the main one being the Gustav Line, that was largely positioned along rivers and mountainous terrain. The Allies had managed to establish a firm foothold in southern Italy and were attacking northwards, while the Germans objective was the defense of Rome.
One of the few access routes to Rome was the Liri river valley, on whose flank was the small town of Cassino and above, on a rocky hill, the imposing Benedictine abbey of Monte Cassino. Although the monastery has remained far more popular, the town of Cassino also saw fierce combats.
By Kirrages (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/deed.en)], via Wikimedia Commons
The first offensive by which the Allies attempted to overcome the Gustav Line began on 17 January 1944. The British Army started promisingly attacking near the Tyrrhenian coast, but the Germans sent two divisions from reserve which blocked the advance. The Americans managed to cross infantry over the Rapido River, without being able to bring tanks on the other side, thus becoming vulnerable when confronted the 15th Panzergrenadier Division.
In the mountains, the Americans came very close to the Monte Cassino abbey on the right flank, but were met with fire by the Germans who had occupied positions at the foot of the monastery. The U.S. Army could not advance more so, in February, a New Zealander Corps, which included British and Indian units, replaced the U.S. troops.
The Germans had promised not to occupy the monastery due to its historical importance. However, on February 15, the Allies suspicion that the monastery was used at least as a lookout for directing artillery fire, although not based on clear evidence, led to a massive aerial bombardment by the U.S. Air Force. After the U.S. bombings, having no longer obligations, soldiers of the German 1st Parachute Division occupied positions among the ruins.
Bundesarchiv, Bild 183-J24116 / Lüthge / [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0/de/deed.en)], via Wikimedia Commons
Shortly afterward, the Allies began the second offensive on the Gustav Line. In the attacks that followed, neither the British, nor the Indians, and not even the Gurkhas, specialized in mountainous terrain, were able to overcome the German paratroopers' defense, the fighting including hand-to-hand combat.
The third offensive began on 15 March, after a powerful bombing that killed almost half of the German troops defending the town of Cassino. The Germans continued, however, to hold out in the north of the town, while the Allies captured only some isolated points in the mountains, not enough to cut the German supply lines.
Finally, the fourth offensive during the Battle of Monte Cassino, known as Operation Diadem, began on 12 May. The British XIII Corps engineers managed to build a bridge across the Rapido River, which allowed the passage of Allied armor. Two Polish Divisions were the last Allied units who attacked the abbey of Monte Cassino but they were also repelled, suffering heavy losses.
However, what would lead to capturing the monastery was not a direct assault on it. The French Expeditionary Corps achieved a critical breakthrough near the coast, into the Aurunci Mountains, so that, on 18 May, the Germans were compelled to withdraw from the Gustav Line, including Monte Cassino, to a line located about eight miles (13 km) further back. In a few days, the new German line was also overcome which, in conjunction with the breakout from Anzio, led to the fall of Rome in the hands of the Allies.
The Allies had obtained a great victory, but the Germans defense for four months and the price paid by the victors have made this battle to enter rightly into legend.