 His treatment of Visby, a member of the Hanseatic League, precipitated that League into war with Denmark; however, though Valdemar was forced into various concessions, he retained Visby as a Danish city.In 1391, 1394 and 1398, it was taken and plundered by the Victual Brothers, pirates who sailed the Baltic Sea.An invading army of Teutonic Knights conquered Gotland in 1398, destroyed Visby and expelled the Victual Brothers.