'Betray' was the British seizure of the island of Kálymnos, between the islands of Kos and Léros, in the Italian-held Dodecanese islands group of the south-eastern part of the Aegean Sea during the first phase of the Dodecanese campaign that followed Italy’s armistice with the Allies on 9 September 1943 (10/17 September 1943).

Brigadier F. G. R. Brittorous’s British 234^th Brigade from Malta, together with 160 men of the Special Boat Service, 130 men of the Long Range Desert Group, A Company of the 11/Parachute Regiment, and a detachment of the Greek Sacred Band secured the islands of Kos, Kálymnos, Samos, Léros, Symi and Astypálaia, supported by ships of the British and Greek navies.

This effort was part of the first phase of the British attempt to secure bases in the Dodecanese islands group after Italy, which had occupied the islands since 1912, reached an armistice agreement with the Allies on 8 September 1943 for implementation on the following day. The British saw the Dodecanese islands group as possible bases against the German-controlled Balkans, but their effort failed as the entire Dodecanese islands group fell to the Germans within two months at the end of a campaign that lasted from 8 September to 22 December and was one of the last major German victories in the war.

The British losses in this campaign were 4,800 men, 113 aircraft, four cruisers severely damaged, four cruisers moderately damaged, six destroyers sunk, two submarines sunk, and 10 minesweepers and coast defence ships sunk. The German losses were 1,184 men and 15 landing craft.

The Dodecanese islands group lies in the south-eastern part of the Aegean Sea, and had been under Italian control since the Italo-Turkish War of 1911/12. During the period of Italian rule, the strategically located islands group became a focus of Italian colonial ambitions in the eastern Mediterranean and Rhodes, the largest of the islands, became a major military and aerial base, and the island of Léros, with its excellent deep-water port at Lakki (Portolago), was transformed into a heavily fortified base for aircraft. After the fall of Greece during April 1941 in 'Marita' and the Allied loss of the island of Crete during May in 'Merkur', Greece and its many islands were occupied by German and Italian forces.

With the ultimate defeat of Axis forces in North Africa in the spring of 1943, Prime Minister Winston Churchill, who at least as far back as the Gallipoli campaign of 1915 in World War I had gained a deep interest in the region, turned his sights on the islands. The British envisaged an operation to capture the Dodecanese islands group and Crete, and thus not only deprive the Axis of excellent forward bases in the Mediterranean, but also apply pressure on neutral Turkey to join the war on the Allied side. This would pave the way, in British thinking, for one of Churchill’s favourite ideas, namely a route through the Dardanelles to Russia as an alternative to the Arctic convoys. At the 'Symbol' conference at Casablanca in January 1943, the initial go-ahead was given, and Churchill ordered his commanders to lay out relevant plans on 27 January 1943.

The core plan was 'Accolade' for a direct attack on Rhodes and Kárpathos using three infantry divisions, one armoured brigade and support elements. The idea of landings at Crete was dropped when it was appreciated that the island was too well fortified and had a strong German garrison. The main problem faced by the Allied planners was the difficulty of countering General Martin Fiebig’s X Fliegerkorps, whose aircraft could operate from bases somewhat closer to the proposed area of operations than those of the Allies, which had to operate from Cyprus and the Middle East. This challenge was further exacerbated by the demands on resources of the forthcoming 'Husky' (i) invasion of Sicily, which was deemed far more important by the Americans.
The Americans were sceptical about the entire concept of an Aegean campaign, which they saw as something serving British post-war political advantages, and an unnecessary diversion from the main front in Italy, which the Americans had entered largely at British insistence. The Americans therefore refused to support the Aegean campaign, warning the British that they would have to rely on their own resources.

As the Italian armistice became increasingly likely, in August 1943 the British began preparing to take quick advantage of a possible split between the Italians and the Germans in the form of a scaled-down 'Accolade'. The assembly of a force based on Major General D. Russell’s Indian 8^th Division was started, and US assistance in the form of Lockheed P-38 Lightning twin-engined long-range fighter squadrons was requested. As a result of the 'Quadrant' first conference in Quebec during August 1943, however, and the US refusal to assent to the British plans, the forces and ships earmarked for 'Accolade' were diverted to other fronts, barely a week before the Italian armistice of 8 September.

When the armistice was announced, the Italian garrisons on most of the Dodecanese islands wished either to change sides and fight with the Allies, or just to return home. In anticipation of the armistice, though, German forces of Generaloberst Alexander Löhr’s Heeresgruppe 'E', based largely on the Greek mainland, were rushed to many of the islands and gained control. The most important German force in the Dodecanese islands was Generalleutnant Ulrich Kleemann’s 7,500-man Sturmdivision 'Rhodos', which had been created during the summer of 1943 on Rhodes, the administrative centre of the Dodecanese islands group and the possessor of three military airfields. Because of this, Rhodes was the principal military objective for both the Allies and the Germans.

On 8 September, the Italian garrison on the island of Kastelorizo surrendered to a British detachment, which was reinforced during the following days by ships of the Allied navies. On the following day a British delegation, headed by Major the Lord Jellicoe, was dropped by parachute on Rhodes with the task of persuading the Italian commander, Ammiraglio di Squadra Inigo Campioni, to join the Allies. Swift action by the German forces, however, pre-empted the Allied effort. Without waiting for the Italians to come to any decision, Kleemann attacked the 40,000-strong Italian garrison on 9 September, and forced it to surrender by 11 September. The loss of Rhodes dealt a critical blow to Allied hopes.

While the new Italian government of Maresciallo d’Italia Pietro Badoglio surrendered and many Italian soldiers in the Aegean were tired of the war and had become opposed to previous regime of Benito Mussolini, Italian Fascist loyalists remained allied to Germany in the Greek campaign, and Generale di Divisione Mario Soldarelli, commander of the 6a Divisione 'Cuneo', rallied Fascist 'Blackshirts' and Italian soldiers loyal to Mussolini to continue the war in support of Mussolini’s aims, and German forces in Greece convinced the 10,000 Italians of the Aegean island garrisons to continue support of their war effort.

Despite this setback, the British high command pressed ahead with the occupation of the other islands, especially the larger islands of Kos, Samos and Léros. The Germans were known to be overstretched in the Aegean, while the Allies enjoyed definite superiority at sea and the air cover provided by two Supermarine Spitfire single-engined fighter squadrons (No. 7 Squadron of the SAAF and No. 74 Squadron of the RAF) over Kos was deemed sufficient. The British hoped that from these islands, and with Italian co-operation, an assault against Rhodes could be eventually launched.

This was the strategic and operational basis on which 'Betray' was planned and executed between 10 and 17 September. In the aftermath of the fall of Kos to the German 'Eisbär' (v), and with most of the British troops already removed to strengthen the forces on more important islands, the Italian garrison of Kálymnos surrendered, providing the Germans with a valuable base for operations against Léros, their next target, in 'Taifun' (iv).