Gen Harold Alexander, the Allied commander-in-chief in the Mediterranean, was exasperated with Gen Mark Clark’s unimaginative and uncoordinated attacks on Monte Cassino, and his failure to capture it despite three attempts and the priority of support in the theatre. Moreover, the landing at Anzio was at a stalemate. It was obvious that the forces at Anzio were not going to come to the rescue of those at Monte Cassino, but those at Monte Cassino needed to break through to come to the aid of those at Anzio. Finally and most importantly, Alexander knew that he would only have one more “go” at Monte Cassino before the priority of men and material went to Operation Overlord, the upcoming invasion of France in May. The fourth battle for Monte Cassino had to succeed or any chance of capturing Rome before autumn, or even winter, would be nonexistent and Germany would be able to shift troops from Italy to resist the invasion of France.
Alexander’s staff produced Operation Diadem, a massive coordinated attack involving all Allied troops in Italy. To gain the necessary mass and concentration, Alexander stopped all operations along the Adriatic coast, and to Clark’s relief, had the British Eighth Army take over the area around Monte Cassino. Alexander then shifted the American 5th Army south west and told Clark to focus on Anzio. The 5th Army units still in the south would nominally be Clark’s but would actually support the Eighth Army whose boundary was extended to the Liri Valley.
Alexander, ever a fan of Lord Horatio Nelson, took to heart Nelson’s quip prior to the Battle of Trafalgar, “Only numbers can annihilate”. He had to do just that in order to prevent the Germans from falling back to the next mountain defensive line and repeating the bloodbath of the last four months. Alexander planned to use entire Allied corps to seize areas that were division objectives in Clark’s operations. The US Second Corps would attack up the Tyrrhenian coast road. French Gen Alphonso Juin’s French Expeditionary Corps would attack over the impassable Aurunnci Mountains that formed the southern shoulder of the Liri Valley. While the British XIII Corps would attack over the northern shoulder. The Canadian I Corps would be in reserve to exploit the breach, and it fell to the 75,000 men and women of the Polish II Corps to seize the Monastery.
An ambitious plan of this size and scope took a month to prepare and required the movement of hundreds of thousands of troops, all of which had to be in secret. On 2 April, 1944, the Allies launched Operation Nunton to deceive the Germans as to the Allies’ preparations over the next five weeks for Diadem. Nunton consisted of thousands of fake radio messages, and dummy vehicle parks and supply depots around Naples in order to convince the Germans that the Allies were planning another amphibious invasion north of Rome. Additionally, Nunton encouraged the Germans to believe that the units to their front were not being reinforced or replaced.
Operation Nunton was wildly successful. The Germans didn’t suspect another assault to open the Liri Valley and had no idea the Allied troops to their front had shifted and were reinforced. For example, Juin’s, 50,000 strong French Expeditionary Corps with their diverse colonial troops and distinctive uniforms had to move from vic Monte Cairo to below the Liri Valley, a distance of 15 miles, and the Germans never suspected a thing. Alexander tripled the number of soldiers in the attack zones for Operation Diadem, set to launch concurrently with Operation Overlord in May.