The planned invasion of the Channel Islands codenamed Grune Pfeile (Green Arrow) had evolved during June 1940.The German High Command was unsure whether or not the islands were defended. Various reconnaissance sorties were flown over the Islands ending with a bombing raid on the harbour facilities of both Guernsey and Jersey. Meeting little opposition it was decided to proceed with the planned invasion. This would involve Stuka dive bombers to soften up the coastal defences followed by landing craft carrying troops armed with light weapons to take the beaches. However before finalizing any arrangements it was decided to send a second armed reconnaissance flight to try and land. If no opposition was met, naval and army units would be flown in. On 30 June 1940 Hauptmann Liebe-Pieteritz, on a routine Luftwaffe reconnaissance flight, anticipated this decision and decided to test the Guernsey defences. Seeing that the airport appeared deserted he landed and found it to be undefended. When news of this reached Luftflotte 3 they concluded that the islands were undefended and were awaiting invasion. A platoon of Luftwaffe troops were flown into Guernsey on Junkers transport aircraft.

The following day more transport aircraft brought naval assault troops, a light anti-aircraft unit, and a company from Infantry Regiment 396. By 2 July radio communications had been set up with the mainland, and the construction of anti-aircraft batteries was begun in Guernsey and Jersey on 4 July. All the existing forts that were initially thought to be a threat were found to be deserted and disused, and the Germans quickly set about putting these to use. The remainder of the early defences were nothing more than earthworks and lightweight structures. Early in 1941 Hitler turned his attention to fortifying the Channel Islands. He feared that once Operation ‘Barbarossa’ (the invasion of Russia, beginning on 22 June) was underway, the British may well attempt to re-capture the Channel Islands. Around this time the Germans began to employ local labour for the construction of its fortifications.

In March 1941 the order went out to strengthen the defences on the Islands and more troops, together with several Navy and Army construction battalions, began to arrive. Reinforced field order fortifications started to appear around the coast. This work was not yet being undertaken by the O.T., but by the Army and Navy construction units. These were mainly units of company strength taken from Construction Battalions, Bridge Construction Battalions, Fortress Construction Battalions, and Railway Construction Battalions (despite the fact that no railways were being constructed). These units arrived in March 1941 and stayed for 3-6 months, after which the O.T. began to arrive. A report dated April 1941 stated that on Guernsey one 22cm battery (Strassburg) was in the process of construction, and would be combat ready by the beginning of May. Work on the early reinforced field order defences continued while the various Festpistab units carried out their surveys. These fortifications were designed to be incorporated into the later defences. A report dated June 1941 stated that the 22cm battery was now combat ready in permanent positions with most ancillary shelters complete while construction of the gun platforms for a 15cm battery was in progress.

In July 1941 Army Coastal Batteries 462 and 463 - each with four 15cm K18 guns, 464, 465, and 466 - each with three 21cm Mrs 18 howitzers, and 471 and 472 - each with four 22cm K532(f) guns were transferred to Guernsey. By the end of the month Batteries 462-466 were combat ready, with the remaining crews being left to wait for their guns to be reconditioned in Germany; five anti-aircraft batteries were also combat ready. In September 1941 Higher Command requested the installation of one battery of 4 x 38cm guns and four batteries of 4 x 15cm SK C/28 guns. By early 1941 Hitler had ordered that the defences of the Channel Islands were to be further strengthened and asked for a full survey to ascertain what was needed to make the islands impregnable. The survey began on July 1941, when Festpistab 19 (Fortress Engineer Staff 19) with Abschnittgruppe I/19 and II/19 (Sector Groups) arrived in Guernsey. Overseeing this operation was Generalmajor Rudolf Schmetzer, Inspekteur der Landbefestigung bei Oberbefehlshaber West (Inspector of Ground Fortifications at Supreme Commander-in-the-West) Early in October 1941 Generalmajor Schmetzer submitted his survey and, two days later following a high level conference in Berlin, Hitler signed his Directive and approved plans to turn the Channel Islands into “Impregnable Fortresses”. It was only after this meeting that the short-term programme (to be completed in 14 months) got underway.

The various strong points around the coastline were first constructed by the troops and later by military construction units. The existing forts were already sited in good strategic positions and these were adapted by the addition of extra defences. Strong points consisted mainly of casemated coastal guns, personnel shelters, searchlight bunkers and anti-tank guns enfilading the beaches. Approval was given for the installation of a 30.5cm battery (Mirus) in place of a proposed 38cm battery and the replacement of the 22cm guns of Batterie Strassburg with 15cm SK C/28 guns. Whilst the 30.5cm battery was constructed, the replacement of the Strassburg guns would never materialise. As early as July 1941 it was apparent that the Fortress Engineers alone would not be able to cope with the scale of the works, and the O.T. should be called in to assist. By November 1941, Dr Fritz Todt had visited the three islands where he issued Construction Orders identifying the following areas of responsibility: 1. Individual troops - field fortifications (trenches, foxholes etc.) 2. Divisional Engineers - distribution of land mines and flame throwers. 3. Army Construction Battalions - reinforced field order defences 4. Fortress Engineers/Construction Battalions - supplying and installing fortress weapons, some tunnelling, transport of heavy loads, compiling construction progress reports and maps, ordering and supervising O.T. tasks. 5. Organisation Todt - quarrying, construction of roads, power stations, most tunnelling, supervising civilian building firms, sea transport, controlling non-military labour and building fortress standard defences.

By early November 1941 considerable amounts of stores and personnel had begun to arrive, and the fortification programme was soon to be underway. On 15 December 1941 Hitler issued the order for the construction of the Atlantic Wall. To oversee this, Generalmajor Schmetzer left the Channel Islands and was replaced by Festungspionierkommadeur XIV under Oberst von Marnitz. Elizabeth College was chosen as headquarters, shared with Festpistab 19 which had been there since the previous July. Subordinate units included - Abschnitt Gr.I/19, Abschnitt Gr.II/19, Festungsbaubtl.19, as well as Rock-Drilling, Mining and Compressor Companies. Despite the large amount of works initially planned for the island, not everything was achieved. Three of the four 15cm SK batteries that were requested never arrived; only Batterie Steinbruch was constructed. Of the seven proposed naval range-finding towers (Marinepeilstande), only four would be built. Likewise, of the six 8.8cm Flak batteries, only two were built to near fortress standard with the other four remaining in field emplacements. Of the many tunnels started on the island, most were left unfinished due to lack of labour and materials. Construction work in the Channel Islands peaked in April 1943. Italy’s withdrawal from the war in September 1943 resulted in thousands of workers being transferred from the Islands to build more fortifications on the Franco-Italian border. Nevertheless, a huge amount was achieved during 1941 to 1943. Set back from the coast on the higher ground were three Naval Batteries, nine Army Coastal batteries (HKAR/1265) and five Army Divisional batteries (Artillery Regiment 379) with guns ranging from 10cm to 30.5cm. Observation posts were constructed inland and on the cliffs. The entire coastline was ringed with resistance nests and strong points, and 54,000 mines had been laid by the beginning of April 1944.In addition substantial headquarters bunkers were built for the various arms of services. This was linked by a fortress telephone network running diagonally across the island from north to southwest. The cable was buried 2 metres deep, and passed through four Netzknotenpunkt (Network junction bunkers), 21 Kabelschaltstellen (Cable switching posts), and a repeater station. By the end of January 1944 the fortifications on the Channel Islands had consumed 484,000 (Festung Guernsey Table lists 613,000cubic metre by 1/9/44) cubic metres of concrete compared with 6,100,000 cubic metres for the rest of the Atlantic Wall. In addition, the amount of excavation work is even more interesting. At the start of 1944 the construction of the Atlantic Wall had necessitated the removal of 255,000 cubic metres, while at the same time 244,000 cubic metres had been excavated in the Channel Islands. Taking into account these statistics alone, the Atlantic Wall could perhaps have been made much stronger had the effort and resources used on the Islands been utilised elsewhere After liberation, the massive clear-up operation began. British troops and German prisoners of war set about clearing the thousands of mines laid around the coast. Weapons of all calibres had to be assembled in various locations prior to being taken out to sea for dumping. The 30.5cm guns of Batterie Mirus were cut up for scrap some years after the war. Gradually most of the coastal fortifications were backfilled and landscaped over. Many field order gun-emplacements were situated on agricultural land, and these were gradually eradicated and the land returned to farming. However, only a small proportion of the structures have been demolished, mostly to make way for re-development. One bunker was removed from the New Jetty in St Peter Port due to fears that weight may collapse the jetty. Several observation posts serving the inland gun batteries were removed. The German Power Station in the Bouet was demolished to make way for a housing estate. The biggest losses were Naval Range-finding Tower MP 1 at Chouet, which collapsed into the quarry in 1991, and Batterie Steinbruch, which apart from two surviving flak bunkers was also destroyed by quarrying. Now some sixty years on feelings have changed, and a growing interest in the occupation of the Islands has meant that there are groups dedicated to restoring the various sites. The Occupation Museum is responsible for the restoration of a 10.5cm Jäger casemate at Fort Hommet and the Naval Range-finding Tower MP 3 at Pleinmont; both are open regularly throughout the season. Guernsey Armouries have restored one of the 22cm emplacements of Batterie Dollmann together with trenches and ancillary bunkers. An original 22cm barrel and replica carriage has been reinstalled. The Occupation Society has also restored the Naval Signals Bunker at St Jacques.