'Grün' (iii) was a German unrealised full-scale operational plan for an invasion of Ireland in support of 'Seelöwe' (summer/autumn 1940).

Despite the detailed nature of its conception, 'Grün' (iii) is believed to have been designed only as a credible threat, and thus as a feint rather than an actual operation for full implementation. The German interest in 'Grün' (iii) and 'Seelöwe' must be considered in light of the Germans' overall strategic plan. Despite the number of German operations launched against the nations of western Europe since 'Weiss' (i) against Poland in September 1939, the attention of the German senior leadership in general, and of Adolf Hitler in particular, was centred on the invasion and destruction of the USSR. The Germans therefore had little real interest in tying major elements of their military capability in France and the UK other than for the task of doing the minimum consonant with the need to prevent the French and British from interfering with the invasion of the USSR. During the period after the fall of France, when the UK faced the possibility of invasion, the Germans were already carefully and secretly concentrating the bulk of their military resources to attack the USSR, which as late as September 1939 had been an ally in the destruction of Poland.

The task of creating and implementing 'Grün' (iii) was the responsibility of General Leonhard Kaupitsch, commander of the German occupation forces in Denmark, but the idea of the operation is believed to have been Generalfeldmarschall Fedor von Bock, commander of Heeresgruppe 'B' and the officer responsible for the western flank of 'Seelöwe'. Some 32 copies of 'Grün' (iii) were distributed on 8 August 1940 to members of the German high command.

'Grün' (iii) was conceived in the early and middle parts of 1940, and was drawn up as a plan during August 1940, fewer than three weeks after Hitler issued his initial warning order for 'Seelöwe' on 16 July 1940. The plan was widely circulated and even publicised in 1940/41, and by 1942 had even made its way into the hands of the Irish military via the British military, and was later translated into English by Irish military intelligence. This is one of the reasons which raised the suspicion that communications 'chatter' about 'Grün' (iii) was permitted in the hope that it would indeed be intercepted by the British and become a 'bogeyman' concept in the minds of British military planners fearful about the exposed western flank of the British Isles, and this is supported by some German sources, which quote an operational instruction issued by the high command on the desirability of misleading the British about a possible invasion of neutral Ireland using all the information media available.

The intention was thus to spread rumours that the Germans were preparing an amphibious assault on Ireland to impose a further stranglehold on the UK as a reinforcement of the existing German air and sea 'siege' of the UK. These efforts may have heightened the state of alert and were a cause of alarm in the UK, persuading the British to make a major effort in seeking to convince the Irish government to abandon neutrality and side with the Allies.

Despite the propaganda, 'Grün' (iii) was a real military plan, and as such was given real consideration. Although Hitler had postponed the implementation of 'Seelöwe' on 17 September, he took a renewed interest in 'Grün' (iii) on 3 December after being told of radio reports alluding to a British invasion of Ireland. Hitler then ordered the staff of Grossadmiral Erich Raeder, the commander-in-chief of the German navy, to investigate the feasibility of occupying Ireland to pre-empt any British attempt to do so. However, at the time Hitler seemed sure that any landing should be undertaken only by Irish invitation.

Raeder and the naval high command were decidedly lukewarm to the concept of 'Grün' (iii), just as they had become to 'Seelöwe'. Raeder’s primary concern concern was for German naval strength, or rather the lack of it, and for the resupply of any army and air force formations which were landed. Raeder said that 'to a defending force, cut off and left to its own devices, the topography of the country offers only limited protection…without supplies and reinforcements they would soon feel the increasing pressure of British expeditionary force brought over under the protection of British naval power; sooner or later our own troops would face a situation similar to Namsos or Dunkirk'.

In this sense 'Grün' (iii) can be seen as a worst-case scenario for the Oberkommando der Wehrmacht. While he was to continue planning and training for 'Seelöwe' and 'Grün' (iii), Kaupitsch seems to have shelved preparations late in 1940 and not returned to them thereafter. From Kaupitsch’s point of view, 'Grün' (iii) had by this time become only a feint. This view is reinforced by examining one of the warnings issued to the German forces involved in the plan: 'The 'Grün' [iii] operation confronts us with an entirely new task. There are therefore no precedents from which we can work. In many cases, troops will have to look after themselves. Each commander must look for a way to achieve his individual objective. Everything depends on the extent of co-operation, on each individual’s alertness and ability to take independent action. Confidence in the achievements of German leadership and the German soldier should be the foundation of this operation.'

Thus the outlook for any German land forces committed to 'Grün' (iii) was poor. They lacked any experience in large-scale amphibious warfare; they would have to fight, without resupply, artillery support and air support, among a hostile citizenry; and they would have to fight against the almost inevitable British military advance from the north of the island, and from the landing of British troops from the mainland of the UK on the flank of the German invasion. While possibly acceptable to Hitler, these and other risks were not acceptable to Raeder in his considered assessment four months after the plan had first been mooted. Moreover, given the current strength of the German forces, Raeder did not agree that Ireland could become a 'back door' into mainland Britain.

As 'Seelöwe' was rescheduled on 12 October 1940 for the spring of 1941, then permanently cancelled on 13 February 1943, 'Grün' (iii) became irrelevant.

Despite the fact that the Abwehr had started to gather intelligence in Ireland only in the middle of 1939, the 'Grün' (iii) plan was thorough in terms of its detail. This can probably be attributed to the use of intelligence gathered by German civilians based in Ireland during the 1930s. That the 'Grün' (iii) plan was completed only days after being ordered is a testament to the planning staff in collating the available data. Hitler’s hope of a détente or non-aggression pact of some kind with the British, whom he considered to be the 'natural allies' of Germany, led him to forbid Abwehr intelligence-gathering in the UK and Ireland during the descent toward war in 1936/38. Even when intelligence-gathering was attempted following the fall of France, it was mostly disastrous. As 'Seelöwe' was postponed and eventually shelved following the launching of 'Barbarossa' against the USSR, the 'Grün' (iii) planning staff issued two later 'editions' of the plan with additional detail.

The full briefing package for 'Grün' (iii) extended to five volumes, each devoted to a particular area of military interest. A good example of the detail the plan included was a 75-page booklet titled Militärgeographische Angaben über Irland (Military Geographical Data on Ireland). This described the frontier, size, historical background, industry, transport infrastructure, vegetation, climate, and weather of the island. It also included 17 pages of detailed sketches of 233 cities, towns and villages, complete with a lexicon. Some 120 photos accompanied the booklet, and annexes contained street maps of 25 cities and towns, including street names and the addresses of garage owners.

A second 'edition' of the plan in October 1941 added 332 photographs (most taken by German tourists) of the Irish country and coast, and these were used to reference highly accurate Ordnance Survey maps. There were also details of spring tides, geological formations and the routes the German troops could use to move inland from projected invasion beaches. Another addendum included in the further reprinting of the plan in 1942 by the Oberkommando der Luftwaffe as the Küsten-Beschreibung des Irischen Freistaates (Irland) (Coastal Description of the Irish Free State) contained high-quality aerial photographs of the areas in question, some taken from altitudes as great as 30,000 ft (9145 m).

Despite this attention to detail, and the improvements in the volume of data with each reprinting, much of the data was obsolete or incomplete. For example, the railway linking Galway and Clifden railway was listed as operational, though it had closed in 1935. Ireland was also described as perfectly suited to military operations because of its 'excellent network of roads', and details on population centres such as Derry and Belfast were accurate but lacked information on British troop concentrations based in these cities. On the other hand, the Ardnacrusha power station on the lower reaches of the Shannon river was entirely detailed in the plan as a result of the fact that the German firm Siemens had built it before the war.

'Grün' (iii) has often been confused with a plan by the Irish Republican Army and sent to the Abwehr in August 1940 and later entitled 'Kathleen' by the Abwehr and 'Artus' by the German foreign ministry. 'Grün' (iii) and 'Kathleen' should not be confused, for the former included no detail of the politics of Ireland, but only military capacity estimates. 'Grün' (iii) made no mention of the IRA in these estimates, and it is clear that even if the planners had wanted to include detail and estimates of the IRA they could not have garnered much in the way of accurate information from the Abwehr.

Leaving aside the plan’s possible propaganda and tactical aims, the military planning aspects of 'Grün' (iii) are best considered as complementary to the aims of 'Seelöwe'. Thus 'Grün' (iii) included among its objectives the need to draw off British army troops stationed in Northern Ireland and who might otherwise be sent to aid the defence of mainland UK; the denial of Ireland as a staging point and/or refuge for British troops; and the provision of a staging post to Luftwaffe forces in subduing the northern UK.

In the event that 'Seelöwe' was successful, the implementation of 'Grün' (iii) was seen as the next step, insofar as operational plans stay static during wartime. 'Grün' (iii) included no plan for the imposition of government in Ireland, but only for the 'rounding up of dissidents', though Dublin was mentioned as one of six German administrative headquarters which were to be established in Britain and Ireland following the successful completion of Sea Lion.

'Grün' (iii) was to launched from the western French ports of Lorient, St Nazaire and Nantes, and to be based on an initial force of only 3,900 men. The objective was to be an 85-mile (137-km) stretch of the south coast between Wexford and Dungarvan. Having captured the ports there, German units were expected to fight their way up to 30 miles (48 km) inland to establish a lodgement extending from Gorey on the road linking Wexford and Dublin across the 2,610-ft (658-m) height of Mt Leinster above Borris, County Carlow, through Thomastown in County Kilkenny, to Clonmel in County Tipperary.

The first landings were to include artillery and commando elements and a motorised infantry battalion. A bridge-building battalion was also to be landed. together with three anti-aircraft companies and several 'raiding patrols' whose task was to probe Irish army defences. Reserves from Generalleutnant Siegfried Hänicke’s 61^st Division, Generalleutnant Helge Auleb’s (from 25 July 1940 Generalleutnant Franz Mattenklott’s) 72^nd Division and Generalleutnant Theodor Freiherr von Wrede (from 19 September Generalleutnant Helge Auleb’s) 290^th Division were to take up occupation duties in the lodgement. Thereafter the details for the plan were sketched rather than fully detailed, and would have depended largely on the success or failure of 'Seelöwe'.

The beach-heads considered in 'Grün' (iii) included the sector between Waterford and Wexford, which was strongly favoured; the estuary of the Shannon river near Limerick; Galway Bay; Donegal Bay with Killala, Ballina and Sligo; Lough Foyle with Londonderry; the 'Bay of Belfast' (Belfast Lough); and Cobh in Cork. The landings were to be effected by sea craft available in occupied France at the time, but there were few in existence and 'Seelöwe' was to have priority, and this was another reason why Raeder was not happy with 'Grün' (iii). 'Grün' (iii) was expected to need more than 50,000 men, and 'Seelöwe' was expected to demand 160,000 men, but for 'Grün' (iii) the Germans found in the north-western ports of France only two ships, in the form of the French Versailles and the German Eule, and three coasting vessels in the form of Mebillo, Clio and Franzine.

It is also worth noting that the passage to Ireland would have required the shipping to pass round the jutting 'toe' of Cornwall in the south-west of the UK. Every vessel involved in 'Grün' (iii) was to carry anti-aircraft weapons, a fact indicating that the planners expected the Royal Air Force to attack the ships, although air cover was to be provided by Luftwaffe warplanes operating from airfields in the north-west of France as part of 'Seelöwe'.

The Germans anticipated that Irish forces would resist the initial invasion, so the landing craft and vessels transporting the German troops were to be equipped with forward-firing guns, and the landing troops were instructed to assume defensive positions as soon as they came under fire, considering retreat only in the direst of emergencies.

There were significant omissions in the German planning: the plans for the proposed assault on Cobh (as a possible beach-head area) was not accompanied by details of the 9.2-in (234-mm) and 6-in (152-mm) artillery defences located there even though these weapons had formed part of the defences of the Treaty Ports which the British had handed to the Irish in 1938.

'Grün' (iii) dealt only with the plan for invasion, and no details of any subjugation of the population and an eventual conquest of the entire island were included. Among the Irish population, however, there was a small element of support for Germany as a result of continued resentment of earlier British rule. There was no involvement or previous knowledge of 'Grün' (iii) by the IRA. It is likely, however, that the possibility of such planning was on the mind of Seán Russell and his acting chief-of-staff, Stephen Hayes. Russell is known to have approached the German foreign ministry and the Abwehr during the time he spent in Berlin, and Hayes is known to have approved 'Kathleen' before it was delivered to the Abwehr in August 1940. However, no operational instructions were issued to Abwehr agents to gather data on Ireland in preparation for 'Grün' (iii) This is possibly because the planners believed that they already possessed sufficient militarily useful data, but also because 'Grün' (iii), although thorough, was created rapidly. Later 'editions' of the plan contained no data from the IRA.