"Operation Diadem, also referred to as the Fourth Battle of Monte Cassino or, in Canada, the Battle of the Liri Valley, was an offensive operation undertaken by the Allies of World War II (U.S. Fifth Army and British Eighth Army) in May 1944, as part of the Italian Campaign of World War II. Diadem was supported by air attacks called Operation Strangle. The opposing force was the German 10th Army.
The object of Diadem was to break the German defenses on the Gustav Line (the western half of the Winter Line) and open up the Liri Valley, the main route to Rome. General Sir Harold Alexander, Commander-in-Chief of the Allied Armies in Italy, planned Diadem to coordinate roughly with the invasion of Normandy, so that German forces would be tied down in Italy, and could not be redeployed to France.
Four corps were employed in the attack. From right to left these were the Polish II Corps and the British XIII Corps, of the Eighth Army, and the French Corps (including Moroccan Goumiers) and the U.S. II Corps, of the Fifth Army. The Fifth Army also controlled the U.S. VI Corps in the Anzio beachhead, some 60 miles northwest."