And now the emperor turned his energies against the panic-stricken King of Denmark. He pursued him from fortress to fortress; attacked him in the open field, and beat him; attacked him behind his intrenchments, and drove him from them through the valleys, and over the hills, across rivers, and into forests; bombarded his cities, plundered his provinces, shot down his subjects, till the king, reduced almost to the last extremity, implored peace. The emperor repelled his advances with scorn, demanding conditions of debasement more to be dreaded than death. The King of Denmark fled to the isles of the Baltic. Ferdinand took possession of the shores of this northern sea, and immediately commenced with vigor creating a fleet, that he might have sea as well as land forces, that he might pursue the Danish monarch over the water, and that he might more effectually punish Gustavus Adolphus of Sweden. He had determined to dethrone this monarch, and to transfer the crown of Sweden to Sigismond, his brother-in-law, King of Poland, who was almost as zealous a Roman Catholic as was the emperor himself. He drove the two Dukes of Mecklenburg from their territory, and gave the rich and beautiful duchy, extending along the south-eastern shore of the Baltic, to his renowned general, Wallenstein. This fierce, ambitious warrior was made generalissimo of all the imperial troops by land, and admiral of the Baltic sea. Ferdinand took possession of all the ports, from the mouth of the Keil, to Kolberg, at the mouth of the Persante. Wismar, on the magnificent bay bearing the same name, was made the great naval depot; and, by building, buying, hiring and robbing, the emperor soon collected quite a formidable fleet. The immense duchy of Pomerania was just north-east of Mecklenburg, extending along the eastern shore of the Baltic sea some hundred and eighty miles, and about sixty miles in breadth. Though the duke had in no way displeased Ferdinand, the emperor grasped the magnificent duchy, and held it by the power of his resistless armies. Crossing a narrow arm of the sea, he took the rich and populous islands of Rugen and Usedom, and laid siege to the city of Stralsund, which almost commanded the Baltic sea.