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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 Free 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.

Diadem was launched at 23:00 pm on 11 May 1944 by elements of the British 4th Infantry Division and 8th Indian Infantry Division, with supporting fire from the 1st Canadian Armoured Brigade. They made a successful strongly opposed night crossing of the Garigliano and Rapido rivers. This broke into the heart of the German defenses in the Liri valley against strong opposition and drew in German theater reserves, reducing pressure on the Anzio beachhead. The Free French Corps pushed through the mountains to the left on 14 May, supported by U.S. II Corps along the coast. On 17 May, Polish II Corps on the right attacked Monte Cassino.

When their position collapsed, the Germans fell back from the Gustav Line to the Hitler Line some 10 miles to their rear.

On 23 May, the four corps attacked the Hitler Line. On the same day, the U.S. VI Corps attacked out of the Anzio beachhead.

The Hitler Line was breached by the 1st Canadian Infantry Division's 4th Princess Louise Dragoon Guards at Pontecorvo on 23 May. The German Tenth Army was forced to retire northwestward. The U.S. VI Corps, moving northeast from Anzio, was at the point of cutting off the German line of retreat when Lieutenant General Mark W. Clark, commander of the Fifth Army, inexplicably ordered them to turn northwest and advance on Rome instead. There is much speculation that he did this so that his Fifth Army would capture Rome ahead of the Eighth Army advancing up the Liri Valley. The German Tenth Army thus avoided being surrounded.

The Germans fought a series of delaying actions, retired to the Trasimene Line, and then to the Gothic Line (identified on German maps as the "Green" Line), north of the Arno River.

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