'Poseidon' was the German occupation of the island of Sámos in the Aegean Sea within 'Zwischenspiel' (12/22 November 1943).

At the time of the Italian armistice with the Allies in September 1943, the British had decided that the major island of Rhodes would be taken (probably by Major General D. W. Reid’s Indian 10^th Division and Brigadier M. G. Roddick’s British 9^th Armoured Brigade provided by General Sir Henry Maitland Wilson’s Middle East Land Forces command) in 'Accolade', but that operation required fighter cover which could only be provided from the three airfields on the island of Kos. It was therefore planned that Kos should be taken as a preliminary to 'Accolade', and that at the same time the islands of Léros and Sámos farther to the north should be occupied, the former as it possesses a good anchorage, and the latter as it covers the approaches to the Turkish port of Smyrna, where Allied troops and equipment would be landed in the event of Turkey entering the war on the Allied side after the successful implementation of 'Accolade'.

It was also decided that these and smaller islands would be ideal bases from which Long Range Desert Group and other raiding organisations could operate against German garrisons on the other islands of the Aegean Sea. By the end of September 2,700 troops of Brigadier F. G. R. Brittorous’s 234^th Brigade had been delivered to these three main islands, and Brittorous had been appointed to command both the Léros garrison and the British forces in the Aegean Sea. However, it soon became clear that Brittorous could not deal adequately with both tasks, and Major General H. R. Hall was appointed on 1 November to command in the Aegean Sea area, with his headquarters on Sámos, and arrived on 11 November.

On the following day the Germans began their 'Leopard' (iii) operation to take Léros, succeeding by 16 November. It was now clear that 'Accolade' could not be launched (especially as Kos had fallen to the Germans on 3/4 October), and that they was little point in maintaining any real presence in the Aegean Sea, and it was decided that the surviving British garrisons should be evacuated, the British departure from Sámos taking place without German interference on 19/20 November.

The Germans were thus able to implement 'Poseidon' without interference: after Junkers Ju 87 dive-bombers of Hauptmann Helmut Naumann’s I/Stukageschwader 3 based at Mégara had attacked their positions, the 2,500-man Greek garrison surrendered on 22 November. The Germans then remained in control of the island until 4 October 1944, when the island was liberated by the Greek forces of the Hieros Lochos (Sacred Band).