Iceland remained part of Denmark, but in keeping with the rise of nationalism around Europe in the nineteenth century, an independence movement emerged. The Althing, which had been suspended in 1799, was restored in 1844, and Iceland gained sovereignty after World War I, becoming the Kingdom of Iceland on 1 December 1918. However, Iceland shared the Danish Monarchy until World War II. Although Iceland was neutral in the Second World War, the United Kingdom invaded and peacefully occupied it in 1940 to forestall a Nazi occupation, after Denmark was overrun by the German Wehrmacht. Due to the island's strategic position in the North Atlantic, the Allies occupied the island until the end of the war, with the United States taking over occupation duties from the British in 1941. In 1944, Iceland severed its remaining ties with Denmark (then still under Nazi occupation) and declared itself a republic. Following the Second World War, Iceland was a founding member of the North Atlantic Treaty Organization and joined the United Nations one year after its establishment. Its economy grew rapidly largely through fishing, although this was marred by disputes with other nations.