[1] When Kalmar King Eric opened the Baltic trade routes for Dutch ships and introduced a new toll for all foreign ships passing the Oresund (Sound Dues), six Hanseatic cities (Hamburg, Lübeck, Lüneburg, Rostock, Stralsund, Wismar) declared war, put a naval blockade on Scandinavian harbors and allied with Eric's enemy, Henry IV, count of Holstein.

Ever since Denmark's defeat in the previous Danish–Hanseatic War, the Hanseatic League held a strong monopoly over Baltic trade.

Denmark had to bestow multiple privileges to the Hanseatic League, one of which being a guarantee of free trade throughout the Baltic Sea, and exemption from tolls.

In 1397, due to the rising power of the Hanseatic League, and particularly German city of Lübeck, the three Nordic kingdoms of Denmark, Norway, and Sweden, put aside their differences to unite under a single monarch.

Under Margaret I, the architect of the Kalmar Union, the Hanseatic League was coerced into returning several forts in Scania that the Hansa had seized per the Treaty of Stralsund.

This was in direct violation of the Treaty of Stralsund, and shortly after, the Wendish cities, led by Lübeck, declared war on the Kalmar Union.

The war started with a Danish victory in the Oresund, and a Hanseatic salt fleet sailing to Prussia was captured by the Danes as well.

These peace agreements weakened Eric's position dramatically, and in 1439 he got dethroned by Danish, Swedish and Norwegian Privy Councils.

The Hanseatic League would be replace him with Christopher of Bavaria, who in turn would be succeeded by Christian I, the first Oldenburg king of Denmark.

Almost immediately into their reigns, Christopher and Christian began favoring native and Dutch traders to counteract the Hanseatic monopoly.