The unified kingdom of Denmark emerged in the 10th century as a proficient seafaring nation in the struggle for control of the Baltic Sea. Denmark, Sweden, and Norway were ruled together under one sovereign ruler in the Kalmar Union, established in 1397 and ending with Swedish secession in 1523. The areas of Denmark and Norway remained under the same monarch until 1814, Denmark–Norway. Beginning in the 17th century, there were several devastating wars with the Swedish Empire, ending with large cessions of territory to Sweden. After the Napoleonic Wars, Norway was ceded to Sweden, while Denmark kept the Faroe Islands, Greenland, and Iceland. In the 19th century there was a surge of nationalist movements, which were defeated in the First Schleswig War. After the Second Schleswig War in 1864, Denmark lost the Duchy of Schleswig to Prussia. Denmark remained neutral during World War I, however, in 1920 the northern half of Schleswig became Danish again. In April 1940, a German invasion saw brief military skirmishes while the Danish resistance movement was active from 1943 until the German surrender in May 1945. An industrialised exporter of agricultural produce in the second half of the 19th century, Denmark introduced social and labour-market reforms in the early 20th century that created the basis for the present welfare state model with a highly developed mixed economy.