Denmark, officially the Kingdom of Denmark, is a sovereign state in Northern Europe, located southwest of Sweden and south of Norway, and bordered to the south by Germany. The Kingdom includes two autonomous constituent countries in the North Atlantic Ocean, the Faroe Islands and Greenland. At 43,094 square kilometres, with a population of around 5.6 million inhabitants, Denmark consists of a peninsula, Jutland, and the Danish archipelago of 407 islands, of which around 70 are inhabited. The islands are characterised by flat, arable land and sandy coasts, low elevation and a temperate climate. The national language, Danish, is closely related to and mutually intelligible with Swedish and Norwegian.The Kingdom of Denmark is a unitary constitutional monarchy, with Margrethe II as queen regnant, organised as a parliamentary democracy. The Constitution of Denmark was signed on 5 June 1849, ending the absolute monarchy which had begun in 1660. The Constitution has been revised four times, most recently in 1953. The seat of government is in the capital of Copenhagen. Denmark exercises hegemonic influence in the Danish Realm, devolving political powers to handle internal affairs. Denmark became a member of the European Union in 1973, maintaining four opt-outs from European Union policies, as outlined in the 1992 Edinburgh Agreement. Both the Faroe Islands and Greenland remain outside the Union.The unified kingdom of Denmark emerged in the 8th century as a proficient seafaring nation in the struggle for control of the Baltic Sea. Denmark was part of the Kalmar Union, established in 1397, ended with Swedish secession in 1523. The following year, Denmark entered into a union with Norway, which lasted nearly three centuries until its dissolution in 1814. Beginning in the 17th century, there were several cessions of Danish territory; these culminated in the 1830s with a surge of nationalist movements, which were defeated in the 1864 Second Schleswig War. Denmark remained neutral during World War I. In April 1940, Denmark was invaded by Germany with brief, military skirmishes while the Danish resistance movement was active from 1943 until the German surrender in May 1945. Denmark joined NATO in 1949, relinquishing its historically neutral stance. An industrialized exporter of agricultural produce in the second half of the 19th century, Denmark introduced social and labour-market reforms in the early decades of the 20th century, making the basis for the present welfare state with a highly developed mixed market economy. The Danish krone has been pegged to the euro since 1 January 1999.Denmark is frequently ranked as the happiest country in the world in cross-national studies of happiness. Denmark ranks as having the world's highest social mobility, a high level of income equality, has one of the world's highest per capita income, and has one of the world's highest personal income tax rates. For 2013, Denmark is listed 15th on the Human Development Index and 9th on the inequality-adjusted HDI. Denmark ranks highly positive on the Corruption Perceptions Index and the Legatum Prosperity Index, and as a full democracy on the Democracy Index. Denmark is among the founding members of the NATO, the Nordic Council, OECD, OSCE, and the United Nations. There are three Danish heritage sites inscribed on the UNESCO World Heritage list in Northern Europe. Greenland, which is part of the Kingdom of Denmark but self-governed since 1979, has one of the highest suicide rates in the world.