'Nunton' was an Allied cover and deception operation, in association with 'Diadem', designed to persuade the Germans that the Allies were planning to execute a major amphibious assault at Civitavecchia to the west-north-west of Rome on the west coast of Italy in the Latium region (March/May 1944).
The object of this deception plan was to persuade the Germans to reduce their strength on the southern portion of the Italian front, where General the Hon. Sir Harold Alexander intended his Allied Armies in Italy command to deliver their 'Diadem' offensive, and also to hold their reserves to the north of Rome. 'Nunton' was based on the concept of suggesting that Alexander, in line with historical precedent under which all successful conquerors of Rome had approached from the north, planned to thin the forces on his main front, where only a holding operation would be conducted, while employing his main forces in a major amphibious assault at Civitavecchia to the north of Rome on 15 May.
The real 'Diadem' offensive was scheduled to open four days earlier, on the night of 11 May, and the Allies therefore hoped that they could persuade Generalfeldmarschall Albert Kesselring, the Oberbefehlshaber 'Südwest', to hold his reserves well back from the Cassino front to meet the supposed threat to Civitavecchia.
'Nunton' was conducted largely by the movement of real forces and the dissemination of false information through special means. The land forces involved were in reality those being held in reserve to exploit the 'Diadem' breakthrough once it came. The movement of divisions to create a concentration of strength behind the Cassino front was explained as movement toward embarkation ports. Lieutenant General Mark W. Clark’s US 5th Army grouped most of its reserves at Naples and conducted vigorous training in that area, and a large quantity of associated radio traffic was generated. Lieutenant General Sir Oliver Leese’s British 8th Army conducted combined exercises at Salerno, again with extensive radio traffic. French forces on Corsica also contributed to the picture generated in German minds that the Allies were preparing a major amphibious operation.
Civitavecchia received attention from air bombardment and seaborne reconnaissance, including a demonstration by 'Beach Jumpers' visual deception teams in 'Spam'.
'Nunton' is generally agreed to have been the most successful Allied deception plan in the Mediterranean theatre after the support accorded to 'Husky' (i) by 'Ferdinand'. By the beginning of May, both Kesselring and the Fremde Heere 'West' intelligence and analysis organisation had decided that there was a 'Naples Group' of four to six divisions in their supposed Allied order of battle. From a time about a fortnight before this, the Oberkommando der Wehrmacht had been noting agent reports of plans for a landing at Civitavecchia, and the Oberkommando der Wehrmacht thought that Civitavecchia was the target of the 'Naples Group' while Kesselring thought that landings at Gaeta or Nettuno, to enlarge the 'Shingle' beach-head at Anzio, were at least as likely.
Kesselring was certain that when the Allied attack on the 'Gustav-Linie' started, it would be only a feint to draw his reserves away from the main amphibious assault, and therefore kept four high-quality divisions in reserve to meet this phantom attack, one each at Civitavecchia, Gaeta and Nettuno, with a Panzer division located in a position to move to the aid of any of them. The Oberkommando der Wehrmacht left Generalmajor Wilhelm Schmalz’s 1st Fallschirmjägerpanzerdivision 'Hermann Göring' to the north of Rome rather than moving it to France as had been planned.
After the battle, almost all the intelligence files of General Joachim Lemelsen’s 14th Army were captured, and these revealed the Germans' complete acceptance of 'Nunton'.