'Alabaster' was the initial British convoy operation to deliver forces for the occupation of Iceland, at the time thought to be under the threat of German occupation with possibly dire consequences for the British convoy routes across the North Atlantic in the event that the Germans were able to establish air and naval bases on the island (17/18 May 1940).

Comparatively large but boasting only a very small population, Iceland had been a sovereign state under the Danish crown since 1918, with its diplomatic and defence needs still met by Denmark in 1939. On the outbreak of World War II Iceland declared itself neutral, and at the time of the German invasion and conquest of Denmark in April 1940 also declared itself temporarily independent of Denmark before, in 1944, finally declaring itself a completely independent republic. The country refused the offer of British protection lest this spur some type of German retaliation, but the dominant strategic position of Iceland in the North Atlantic was too important for the UK to ignore the possibility of a German occupation of the country, no matter how unlikely this might be.

The British landing of May 1940 was therefore designed to pre-empt any German undertaking, but the British promised not to interfere in any way in Iceland’s internal affairs. The British interest in Iceland had developed from a time early in the war within the context of the UK’s economic and military reliance on convoys plying east/west and north/south through the North Atlantic. The westward development of British naval support in general and close escort in particular for the Atlantic convoys was by 1940 determined by the facility with which naval and air bases could be completed in Iceland. The anxiety of the British war cabinet about the security of Iceland and also of the Færoe islands group, another Danish dependency, was already considerable, for there were fears that if these islands were taken by the Germans and turned into sea and air bases, the threat to the British lines of communication across the Atlantic would become acute.

As it became clear that 'Weserübung' had taken Denmark and would shortly succeed in taking Norway, the British authorities therefore moved with some speed. On 8 May an advance party of the Royal Marines ('Sturges' Force commanded by Colonel Robert Sturges and centred on the 2/101^st Royal Marine Brigade including three batteries of artillery and 815 men plus a small intelligence detachment) departed Greenock in the heavy cruiser Berwick and light cruiser Glasgow for Iceland, this element reaching Reykjavik two days later in 'Fork'. The squadron searched the eastern fjords for any signs of German activity, and returned to its base with all the German nationals found on Iceland. A week later two large transport vessels delivered an infantry brigade to Iceland in 'Alabaster', and on 23 May the transport Ulster Prince brought an army detachment to Tórshavn in the Færoe islands group, where Royal Marines had already landed in 'Valentine'.

In the middle of June the first of several reinforcements of Canadian troops were carried direct from Halifax to Iceland, and small garrisons were established in the various fjords which might have offered the Germans a toehold. Early in July reinforcements of Canadian troops from Halifax and of British troops from the Clyde were taken to Reykjavik, and measures to defend possible landing places against German assault were set in motion. An anti-submarine boom was started in Hvalfjördur, a short distance north of Reykjavik, where the main naval base was to be set up. The construction of the shore installations from nothing was, of course, a slow process.

Similar undertakings were launched at Tórshavn in the Færoe islands group and Stornoway in the Outer Hebrides islands group, where bases were to be constructed for improved air cover over the large quantities of shipping now passing round the north of Scotland. It is worth noting that the requirement to extend the protection of Atlantic convoys against U-boats farther to the west had become clear even before the fall of France in June 1940, but the defeat of France in June 1940, only six weeks after the surrender of Norway on 3 May, exacerbated the British problem by providing the German navy with bases along the western coasts of France as well as Norway.

Speedy improvement of the UK’s ability to convoy merchant shipping were now needed urgently as the passage of U-boats from the French bases to the convoy routes was now some 450 miles (725 km) shorter than from the German ports in which the boats had previously been based. This gave the boats a radius extending deeper into the Atlantic, and allowed the use of smaller U-boats, designed for coastal operations and therefore possessing shorter ranges, in the oceanic role. All the U-boats would, at least for a time, have the benefit of easier and safer transits to their operational areas than through the more closely watched northern exits from the North Sea. So while in the previous phase the German navy could keep 14 out of an operational total of 33 U-boats at sea at any one time, it could now enlarge number to 16 boats out of a smaller total strength.

Despite the decline in the effective strength of U-boat fleet, to a low of 21 during February 1941, the overall capability of the U-boat arm was in fact increased by the use of Norwegian and French bases. July saw the diversion of British shipping from the South-Western Approaches to the North-Western Approaches, the first U-boat base on the Atlantic coast was brought into service at Lorient. The Admiralty assessed the total of completed U-boats at 71 and believed that 24 had been lost since September 1939: in fact 25 had been lost but 51 were still available to the Germans.

On 17 May the three battalions of Brigadier G. Lammie’s 147^th Brigade of Major General H. O. Curtis’s 49^th Division arrived to relieve the Royal Marines in Iceland, and two days later the Royal Marine battalion sailed back to the UK. The defence was stretched very thin in an effort to hold the principal areas on which any German invasion, amphibious or triphibious, might seek to land round Iceland’s long and difficult coast. The British were, of course, unaware of the fact that Adolf Hitler had no real intention of mounting any such undertaking, although limited planning was undertaken. At the urgent request of Lammie, in June the British authorities agreed to send a second brigade to garrison its new outpost, and the three battalions of Brigadier C. G. Philips’s 146^th Brigade arrived with the cut-down headquarters of the 49^th Division, on 27 June. More reinforcements which arrived during the summer of 1940 included field artillery, anti-aircraft guns, Bren gun carriers, engineer and construction units, and support forces.

As early as 18 May the British government had suggested to the Canadian government that Iceland should be garrisoned by Canadian troops. In particular, one battalion was urgently required to reinforce the 147^th Brigade at that time. By June the British were requesting a full brigade of reinforcements and by July it was suggested that the whole of Major General V. W. Odlum’s Canadian 2^nd Division should be despatched to Iceland. Canada sent 'Z' Force under Brigadier L. F. Page with a headquarters staff and one infantry battalion, which arrived on 16 June. Two more Canadian infantry battalions arrived on 9 July, raising the British and commonwealth garrison of Iceland to the equivalent of a composite division.

Canada desired to concentrate its overseas army in one locale under Canadian command, however, and thus 'Z' Force was soon replaced by British forces. Brigadier P. Kirkup’s 70^th Brigade sailed from the UK on 21 October, arriving in Iceland on 25 October and opening the way for the withdrawal to the UK of most of 'Z' Force on 31 October, with the last Canadian battalion following in April 1941. The final British ground reinforcements for Iceland arrived during June 1941 in the form of another infantry battalion and one more battery of artillery. By July 1941 there were thus more than 25,000 British troops on the island.

Construction of naval facilities in Hvalfjörður had started soon after the occupation, and these gradually grew into an important complex which was later enlarged still further to include a major fuel tank farm, minefield, anti-submarine nets, gate and a boom across the fjord, coastal artillery, anti-aircraft batteries, and a flotilla of anti-submarine trawlers. As such, it served as base for Allied escort and anti-submarine forces.

The British air strength on the island was also increased. The Fleet Air Arm’s No. 701 Squadron was the first air unit to arrive after the British landing, but was soon replaced by the RAF’s No. 98 Squadron with 18 aircraft. These were only Fairey Battle single-engined light bombers, and therefore represented no threat to the occasional German reconnaissance aircraft which flew over the island, strafing military camps at least once. No. 1423 Flight of Hawker Hurricane single-engined fighters was consequently despatched in June 1941 but withdrawn in December following arrival of the USAAF' 33^rd Pursuit Squadron. As basing facilities were built up, most of the air units stationed on the island were Coastal Command aircraft for the maritime reconnaissance, patrol and anti-submarine roles.

Iceland opened a legation in New York City after the German invasion of Denmark. Concerned about the ability of the UK to survive following the German defeat of France, in July 1940 the legation approached the US Department of State about the possibility of protection under the Monroe Doctrine, but no action followed. Even so, Iceland began to appear in US planning and, according to the 'ABC-1' staff agreement between the UK and USA, in the event of an American entry into the war US troops would assume responsibility for the defence of Iceland. In April 1941 discussions with Icelandic representatives were reopened by Sumner Welles and Harry Hopkins, the US Under-Secretary of State and presidential adviser respectively.

As the USA became more closely aligned with the UK under the impact of world events in general and the effect of the U-boat campaign on American shipping in particular, President Franklin D. Roosevelt met Lord Halifax, the British ambassador, on 28 May and offered to assume responsibility for the defence of Iceland. Prime Minister Winston Churchill, anxious to draw the USA into the war against the Axis by any and all means, accepted the offer with alacrity and without reservation. The USA still had a strong isolationist movement, however, and in the tricky task of occupying a neutral state the Roosevelt administration felt that it needed a specific invitation from the government of Iceland. The USA was itself still a neutral, but the neutrality and sovereignty of Iceland meant any invitation had to be couched in careful terms with 15 conditions including full recognition of Iceland’s independence and a promise to withdraw 'immediately on conclusion of the present war'.

On 24 May 1941, the 6^th Marines in California became the nucleus of a force 'for overseas duty', and sailed from San Diego on 31 May to reach Charleston, South Carolina, on 15 June. On the following day the 1^st Marine Brigade (Provisional) came into being under the command of Brigadier General John Marston. On 22 June, even as the Germans invaded the USSR in 'Barbarossa', the 4,400 or so men of the 1^st Marine Brigade (Provisional) sailed for the Newfoundland port of Argentia in four transport and two cargo vessels. The convoy reached Argentia on 27 June and awaited the conclusion of negotiations. The US, British and Icelandic governments resolved the diplomatic issues and the relevant invitation was issued by the Icelandic authorities on 1 July. Under the overall supervision of Admiral Harold R. Stark, the US Navy’s chief of naval operations, the marine brigade departed Argentia for 'Indigo' during the following morning in the military transports Heywood, Fuller, William P. Biddle and Orizaba, the cargo ship Arcturus, the destroyer tender Hamul, the oiler Salamonie and the fleet tug Cherokee with a heavy escort including the battleships New York and Arkansas, the cruisers Brooklyn and Nashville, and the destroyers Lea, Upshur, Bernadou, Ellis, Buck, Benson, Mayo, Gleaves, Niblack, Lansdale, Hilary P. Jones, Charles F. Hughes and Plunkett.

This Task Force 19 reached Reykjavik on 7 July. Although the agreement called for the prompt relief of British forces and US Marine brigade by elements of the US Army, the despatch of US infantry was delayed by shortages of equipment and trained personnel, and also by federal legislation prohibiting conscripted personnel from serving outside the USA. Eventually Major General Charles H. Bonesteel’s (later Major General Cortlandt Parker’s) 5^th Division was chosen as the Iceland garrison formation, and on 27 July the first US Army troops sailed in two elements from New York and Norfolk, Virginia. The personnel and 30 aircraft of the 33^rd Pursuit Squadron were transported by the fleet carrier Wasp and, although not carrier-qualified, the pilots flew their aircraft off the carrier’s flightdeck just before the convoy reached Reykjavik on 6 August.

The 5^th Division’s second echelon sailed from New York on 5 September in the transports Heywood, William P. Brook, Harry L. Lee and Republic, reaching Reykjavik on 15/16 September in 'Indigo III'. More units of the US Army deployed to Iceland during 1942.

As the men and equipment of the US Army began to arrive, the British and US Marine units departed. The 70^th Brigade with its three battalions returned to the UK in December 1941. The 3/6^th Marines embarked on 28 January 1942, steamed away on the last day of the month, and arrived in New York on 11 February. The remainder of the US Marines began embarking on 4 March 1942, sailed on 8 March, and arrived in New York on 25 March. The 1^st Marine Brigade (Provisional) was thereupon disestablished.

In April 1942 the 147^th Brigade and HQ elements of the 49^th Division were withdrawn, leaving only the 146^th Brigade and assorted support and administrative forces to represent the UK on the island. In the summer of 1942, most of these sailed for the UK, the 146^th Brigade on 20 August. By the summer of 1943 the last British troops had gone, but the strength of the Royal Navy and RAF remained, and their activities continued unabated.

In 1943 the tide of war turned decisively in favour of the Allied powers, and with this event there disappeared any realistic threat of German intervention on Iceland. The 5^th Division (2^nd, 10^th, and 11^th Infantry), as well as numerous support units and the 759^th Light Tank Battalion, left Iceland on 5 August 1943 and reached the UK on 9 August. The separate 118^th Infantry left the island on 29 October and reached the UK on 6 November 1943. Despite the departure of all major ground combatant elements, there remained on Iceland a substantial strength of US anti-aircraft, coastal artillery, engineer, and support troops, and also major air and naval forces.

In 1945 the last Royal Navy elements were withdrawn, and the last units of the RAF in March 1947. Despite the original Iceland invitation, with its 15 conditions, some of the US forces remained on Iceland after the end of hostilities, and in 1946 an agreement was signed granting the USA use of military facilities on the island.