Danish–Icelandic Act of Union in the context of Icelandic constitutional referendum, 1944

A constitutional referendum was held in Iceland between 20 and 23 May 1944. The 1918 Danish–Icelandic Act of Union declared Iceland to be a sovereign state separate from Denmark, but maintained the two countries in a personal union, with the king of Denmark also being the king of Iceland. In the two-part referendum, voters were asked whether the union with Denmark should be abolished, and whether to adopt a new republican constitution. Both measures were approved, each with more than 98% of valid votes in favour. Including invalid ballots, voter turnout was 98% overall, and 100% in two constituencies: Seyðisfjörður and Vestur-Skaftafjellssýsla.

The Kingdom of Iceland (Icelandic: Konungsríkið Ísland; Danish: Kongeriget Island) was a sovereign and independent country under a constitutional and hereditary monarchy that was established by the Act of Union with Denmark signed on 1 December 1918. It lasted until 17 June 1944 when a national referendum established the republic of Iceland in its place.

Under a personal union, due to the Act of Union, the monarch was simultaneously monarch of Denmark. The Parliament of Iceland asked that Denmark represent Iceland internationally, and day-to-day matters were delegated to a Danish plenipotentiary for Icelandic affairs based in Reykjavík, and – after the German invasion of Denmark in 1940 – a regent was appointed.

On 10 May 1940, during World War II, the United Kingdom landed Royal Navy and Royal Marines forces at Reykjavík and occupied Iceland without resistance. The operation, codenamed Operation Fork, was launched after Germany's rapid conquest of Denmark, to which Iceland was linked by a personal union, and amid British fears that Germany might establish bases on the strategically important island. Iceland had declared neutrality at the start of the war and rejected British requests for cooperation, but the UK proceeded with an unannounced landing to pre-empt any German move.

British troops quickly secured Reykjavík, detained German nationals, and occupied key communication and transport sites before spreading out to potential landing grounds and harbours. Iceland issued a formal protest against the violation of its neutrality, while Britain promised compensation, non-interference, and eventual withdrawal. Over the following months the initial marine detachment was replaced by British, Canadian, and later American forces, whose presence—eventually numbering tens of thousands—had lasting military, economic, and social effects on the country.