The opera Waldermarsskatten is a fantasy around a real historical event on the island of Gotland on the east coastof Sweden, “The Valdemar Atterdag holding Visby to ransom, 1361”. The trade in the Baltic Sea was dominatedby the German trading monopoly The Hanseatic League, which existed around 1150–1650. The city of Visby wasincluded in this association.
After a plague pandemic in the Nordic countries in the 1350s, the so-called ‘black death’, in which a third of the population died, the Danish king Valdemar Atterdag wanted the rich merchants of the Hanseatic League to pay for a renewal of their trade privileges. Valdemar set off to Visby with about thirty ships to Gotland at the end of July 1361. Battles were fought, first on the coast-line then at Visby’s city wall, and in total around 1 800 Swedes are said to have lost their lives, while the Danes’ loss was around 300 souls. An unfair fight, one might think, as the Danish army consisted of German mercenaries, while Gotland was defended by civilian farmers.
Johan Andreas Hallén was a Swedish Romantic composer, conductor and music teacher, primarily known for his operas, which were heavily influenced by Richard Wagner’s music dramas. Hallén was born in Gothenburg and died in Stockholm, but the early years of his career and most of his education were in Germany. Like his Norwegian contemporary Edvard Grieg and many other composers the same generation, Hallén frequently evokes the folk music and folk stories of his home country in his compositions.
According to the musicologist Axel Helmer, however, "The salient feature of his style [...], and the one which strongly affected contemporary reaction, is its close, almost derivative relationship to German music," especially Wagner.
Around 1885, Hallén returned to Sweden and continued to conduct and compose, and in later years taught composition at the Stockholm Conservatory. https://en.wikipedia.org/wiki/Andreas_Hallén