Whenever money and trade develop in an area, there will be a struggle to control it. This is exactly what happened in the German Cities along the Baltic and North Seas. It did not take long for the cities of Bremen, Hamburg, and Lubeck to join forces and form the Hanseatic League. Think of a twist between modern labor unions and modern gangsters. That's right, when local leaders banned members of the League from trading in their cities, the League refused to trade with that city at all. This made life for the inhabitants of the city difficult, and they would put pressure on their ruler to reinstate the League's right to trade. The Hanseatic League even went so far as to wage war in order to increase their power. Check out the map below to see just where the League operated.

Source: Extent of the Hensa, OwenBlacker, Wikimedia