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How did Valdemar IV's capture of Visby threaten the Hanseatic League?

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Hanseatic League Hanseatic port of Hamburg, manuscript illumination from the Hamburg City Charter of 1497.

When King Valdemar IV Atterdag of Denmark captured Visby in 1361, it signaled that the very foundations of the Hanseatic League’s power were crumbling.1

Visby had long served as a principal northern European mercantile center and a leading city within the league. Valdemar’s expansionist policies, which also included the annexation of Skåne and Öland, granted Denmark complete mastery over The Sound and control of the lucrative Skåne herring industry. By seizing these strategic territories, Valdemar directly threatened the economic dominance and trade security of the Hanseatic cities.21

This aggression forced the league to abandon its internal divisions and organize a common military response, eventually leading to the Danish War (1368–70) and the decisive Peace of Stralsund, which restored and solidified Hanseatic influence.1