Denmark could tax shipping passing between the Baltic and North seas through the Danish straits. That revenue supported Danish sovereignty and constrained other powers. A canal across Holstein would create an alternative route, but its usefulness depended on removing the land from Danish control. The later Kiel Canal could shift warships between seas and bypass a Danish-controlled commercial chokepoint. Prussia and Austria consequently had interests in the duchies that extended beyond German nationalism.
Notable Point · 13:21-14:09
Straits tolls and the strategic logic of Kiel
Danish tolls on Baltic trade and the prospect of a canal around them made control of Holstein important to Prussian and Austrian strategy.