Inspired by some reading on the Thirty Years War and the Danish intervention...
By 1627, Danish intervention in the war had become a disaster, as Habsburg troops overran Holstein and Jutland itself. Protestant forces in Lower Saxony had collapsed, and the Danish Council of State was willing to make peace at any price. This would have, of course, been disastrous for the Protestant cause.
"By now the emperor, and through him Wallenstein, had set his sights on a more valuable prize: the Baltic itself. Both branches of the Habsburg dynasty, the Spanish as well as the Austrian, had been contemplating the possibility of establishing a naval presence along the Baltic’s German shore. Once Imperial and League forces conquered Denmark’s possessions in Lower Saxony, and took control over Christian’s former ally Mecklenburg, this so-called ‘Baltic design’ took form. Construction of an Imperial fleet began at Wismar, Greifswald, Rostock, and even in the Jutish port of Ålborg at the end of 1627. This was no trivial enterprise: together, these shipyards could assemble a considerable fleeet in a very short time. Combined with the Polish fleeteet and Habsburg naval forces from Dunkirk, the Imperial navy in the Baltic would constitute a real challenge to the Denmark, Sweden, the States General, and England." Denmark, 1513-1660: the rise and decline of a Renaissance monarchy
Against the advice of his, err, advisors, the Danish king Christian continued the war, and used his naval superiority to good effect, while he raised the spectre of Scandinavian collaboration by meeting with Gustav Adolphus to discuss an alliance. Ultimately, the idea failed as Sweden intervened in the war, and Imperial forces discerned much bigger problems.
But could Wallenstein's gambit have succeeded? And what would hve been the effects of the Empire ruling the Baltic ports?