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 the Kalmar Union, established in 1397 and ending with Swedish secession in 1523. Denmark and Norway remained under the same monarch until outside forces dissolved the union in 1814. The union with Norway made it possible for Denmark to inherit the Faroe Islands, Iceland, and Greenland. Beginning in the 17th century, there were several cessions of territory to Sweden. In the 19th century there was a surge of nationalist movements, which were defeated in the 1864 Second Schleswig War. Denmark remained neutral during World War I. 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.