How did the union of Poland and Lithuania weaken Hanseatic interests in the eastern Baltic region?

The union of Poland and Lithuania in 1386 significantly harmed Hanseatic interests by disrupting the league's influence in the eastern hinterland.1

The Hanseatic League’s power was fundamentally built upon its control of trade between the industrial west and the "colonial" lands of eastern Europe, which supplied essential raw materials and food surpluses. By consolidating political power in the region, the union challenged the German monopoly over these vital trade routes and resources. This shift was part of a broader decline for the league, as political developments in bordering nations and the rise of competing mercantile powers—such as the English and the Dutch—gradually eroded the Hanseatic League's dominance in the Baltic region.21