'Ikarus' was a German unrealised preliminary plan for the invasion and seizure of Iceland in combination with 'Seelöwe' (June 1940/42).
From a time late in February to early in March 1939, the German navy undertook a major war game posited on the notion of armed conflict with France and the UK. This war game included the idea of an amphibious assault right at the start of hostilities, the likeliest target being Iceland to provide Germany with a strategic base in the Atlantic Ocean. The war game was followed by discussions between army, navy and air force staff officers, and these came to the conclusion that such an undertaking was logistically impossible for Germany to launch, let alone sustain even if the initial lodgement could be secured.
On 11 October 1939, some five weeks after the outbreak of war with France and the UK, Konteradmiral Karl Dönitz, the Befehlshaber der Unterseeboote, noted in relation to the pressing need for a supply ship to support the U-boats in the Atlantic that the 2,452-ton steamer Sandhörn was being rebuilt as Ammerland for this task, possibly for clandestine deployment to Iceland, at the time a possession of neutral Denmark.
After the German seizure of Denmark and Norway in 'Weserübung' from April 1940, Germany’s naval position vis-à-vis the UK and France was materially improved in and round the North Sea, and by 16 May the naval staff had completed a study of 'Ikarus' in all essential detail for a possible occupation of Iceland. The study looked at ways to win and hold air and naval bases on Iceland and how best to use these bases to dominate a battle of the transatlantic sea lanes to encompass the defeat of the UK and France by means of a maritime blockade.
On 20 June 1940, with France on the brink of defeat after 'Sichelschnitt' and as a result of 'Rot', Grossadmiral Erich Raeder, the commander-in-chief of the German navy, presented Adolf Hitler with the results of the study and further naval thoughts about a landing on Iceland. This thinking suggested that the entire German navy would be required for the enterprise, but that ultimately the island could not be held against the supremacy of the British navy, which could and would prevent the reinforcement and resupply of any garrison.
However, this German naval thinking had already been overtaken by 'Alabaster' when, on 10 May, the British landed the first of an eventual 25,000 troops on neutral Iceland. The matter was of little consequence to Hitler because he hoped to reach a solution with the UK, either as a result of a successful invasion in 'Seelöwe', whose preparation he ordered on 16 July, or the success of secret negotiations, which failed, from September via a neutral intermediary. Even so, on 20 November 1942 the naval war diary recorded that Hitler had ordered an investigation of how Iceland might be occupied through the use of transport as the island was then occupied only by US forces, who had replaced the British in 'Indigo III'. In this request Hitler revealed his descent into unreality in the belief that a few hundred lightly armed German troops might be able to capture the island from the US forces stationed there, and then hold it against the inevitable massive US and British countermeasures.