Wallenstein now began to assume the title of generalissimo of the Emperor by sea and land. Wismar was taken, and a firm footing gained on the Baltic. Ships were required from Poland and the Hanse towns to carry the war to the other side of the Baltic; to pursue the Danes into the heart of their own country, and to compel them to a peace which might prepare the way to more important conquests. The communication between the Lower German States and the Northern powers would be broken, could the Emperor place himself between them, and encompass Germany, from the Adriatic to the Sound, (the intervening kingdom of Poland being already dependent on him,) with an unbroken line of territory. If such was the Emperor's plan, Wallenstein had a peculiar interest in its execution. These possessions on the Baltic should, he intended, form the first foundation of a power, which had long been the object of his ambition, and which should enable him to throw off his dependence on the Emperor. To effect this object, it was of extreme importance to gain possession of Stralsund, a town on the Baltic. Its excellent harbour, and the short passage from it to the Swedish and Danish coasts, peculiarly fitted it for a naval station in a war with these powers. This town, the sixth of the Hanseatic League, enjoyed great privileges under the Duke of Pomerania, and totally independent of Denmark, had taken no share in the war. But neither its neutrality, nor its privileges, could protect it against the encroachments of Wallenstein, when he had once cast a longing look upon it. The request he made, that Stralsund should receive an imperial garrison, had been firmly and honourably rejected by the magistracy, who also refused his cunningly demanded permission to march his troops through the town, Wallenstein, therefore, now proposed to besiege it. The independence of Stralsund, as securing the free navigation of the Baltic, was equally important to the two Northern kings. A common danger overcame at last the private jealousies which had long divided these princes. In a treaty concluded at Copenhagen in 1628, they bound themselves to assist Stralsund with their combined force, and to oppose in common every foreign power which should appear in the Baltic with hostile views. Christian IV. also threw a sufficient garrison into Stralsund, and by his personal presence animated the courage of the citizens. Some ships of war which Sigismund, King of Poland, had sent to the assistance of the imperial general, were sunk by the Danish fleet; and as Lubeck refused him the use of its shipping, this imperial generalissimo of the sea had not even ships enough to blockade this single harbour.