Norway traces its proud history to the Vikings, whose legendary seafaring skills took them to almost every place reachable by water. Before the year 1000, the norse had traveled on sea voyages as far as Greenland, which was colonized then. Christianity came relatively late (the beginning of the 11th century) and soon, Norway had achieved the apex of its power. In 1397, all the Scandinavian kingdoms were united under Danish control. At that point, Norway per se ceased to exist as an independent entity -- Danish rule would continue for another 400 years. During the early 19th century, Norway tried to reestablish its own sovereignty and in 1815, Sweden conceded an independent Norway in union with Sweden. This union scheme was never popular and in 1905 the Norwegian Storting pronounced the union over; the Swedish king was replaced with a prince of Denmark, who ruled Norway for over half a century as Haakon VII. Though Norway had been neutral during World War I, and tried to remain so during the Second World War, Germany invaded in 1940. A government-in-exile was established in London. Resistance to the collaborationist government of Vidkun Quisling was strong and with the Nazi exit in 1945, the King and his government-in-exile were returned victorious. Post war, Norway became an original member of the United Nations. A Norwegian, Trygve Lie, was the first Secretary General of the organization. It became a member of NATO and the European Free Trade Association. Though Norway provided its citizens with a model society in many ways, it also faced the burdensome economic consequences of such a society but has continued to enjoy, for the most part, enviable stability and prosperity. Discovery of oil and gas in adjacent waters in the late 1960s boosted Norway's economic fortunes. In referenda held in 1972 and 1994, Norway rejected joining the EU. Key domestic issues include immigration and integration of ethnic minorities, maintaining the country's extensive social safety net with an aging population, and preserving economic competitiveness.