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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 Tenth 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. British General Sir Harold Alexande, the Allied commander in the Italian theatre, 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 Polish II Corps and British XIII Corps, of Eighth Army, and the Free French Corps (including Moroccan Goumiers) and U.S. II Corps, of Fifth Army. Fifth Army also controlled U.S. VI Corps in the Anzio beachhead, some 60 miles northwest.
Diadem was launched at 2300 Hours on 11 May 1944. XIII Corps forced a crossing of the Rapido River at the mouth of the Liri valley against strong opposition, drawing in German local reserves. 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.
The German position collapsed, and the Germans fell back from the Gustav Line to the Hitler Line some 10 miles to their rear. This fighting drew German theater reserves into the Liri valley, reducing pressure on the Anzio beachhead.
The Hitler Line was breached by 1st Canadian Infantry Division's 4th Princess Louise Dragoon Guards at Pontecorvo on 23 May. German Tenth Army was forced to retire northwestward. U.S. VI Corps, moving northeast from Anzio, was on the point of cutting the German line of retreat, when the U.S. Fifth Army commander U.S. Lieutenant General Mark W. Clark inexplicably ordered them to turn northwest and advance on Rome instead. There is much speculation that he did this so that Fifth Army would capture Rome ahead of Eighth Army advancing up the Liri Valley. The German 10th Army thus avoided being surrounded.