This is new research on a war with seldom published English accounts, detailing the military systems of Sweden and Denmark-Norway. The book describes and analyses the Kalmar War of 1611-1613 between Sweden and Denmark-Norway. Since Denmark controlled the Straits between the Baltic Sea and the North Sea, Sweden sought an alternative trade route through the sparsely populated Arctic Lapland - an option enabled by the 1595 Treaty of Teusina (Tyavzino) with Muscovy. In 1607, King Charles IX of Sweden declared himself King of the Lapps and sent men to collect taxes in what by tradition was regarded as Norwegian territory. In response, King Christian IV of Denmark and Norway in 1611 declared war upon Sweden and invaded Swedish territory from the south. Operations also took place along the Swedish-Norwegian border and elsewhere in the Baltic region, on land and on sea. In addition, Sweden employed Scottish privateers and enlisted a Scottish corps which landed in Norway. Nonetheless, Denmark ultimately won the war. However, the Danish thrust against the Swedish heartland failed, and the Kalmar War was the last time when Denmark successfully defended its control of the Baltic Sea against Sweden. The principal commander on the Swedish side was the young Gustavus Adolphus, who later rose to prominence in the Thirty Years' War. The losses in the Kalmar War proved to Gustavus Adolphus that the Swedish army was becoming obsolete and needed thorough modernization. As a result, the defeat in the Kalmar War was the catalyst that prompted Gustavus Adolphus to reform the Swedish army, which in turn set Sweden on the path to become a regional great power in the subsequent Thirty Years' War. Michael Fredholm von Essen presents new research on a war previously seldom described in English. Moreover, the book details the military systems of Sweden and Denmark-Norway in the early seventeenth century and explains the development of the Swedish Army and the military thought of Gustavus Adolphus before he used his reformed army with great success in the Thirty Years' War.​