Wallenstein's thoughts, however, were more immediately directed to the towns on the Baltic. He had long been alarmed at the danger which threatened him from Sweden. In November, 1627, he had entered into negotiations with an adventurer who offered to set fire to the ships in the Swedish harbours. But as the project had broken down there was nothing for it but to gain possession of the port towns on the Baltic coast, and to bar them against the enemy. For no man could expect that Gustavus would look on quietly, whilst a great military power was forming on the southern coast of the Baltic. [Sidenote: § 11. Growth of his power.] Wismar was soon in Wallenstein's hands. The harbour of Rostock was blocked up by a line of sunken ships. Though Boguslav, the Duke of Pomerania, promised to keep his long line of coast safe from attack, he was compelled to admit a strong imperialist force within his territory. Everything seemed to be succeeding as Wallenstein wished. SECTION III.--_The Siege of Stralsund._ [Sidenote: § 1. Stralsund holds out.] One town alone held out. Stralsund was not a free city of the Empire. But though it was nominally dependent on the Duke of Pomerania it was practically its own mistress. The citizens had no wish to put themselves forward in opposition to the Emperor, far less to assist a foreign power to gain a footing in Germany. But they would never admit a garrison of such troops as Wallenstein's within their walls. [Sidenote: § 2. He orders the siege to be commenced.] Wallenstein would have all or nothing. He ordered his commander in those parts, the Lutheran Arnim, to enforce submission. "I will never," he wrote, "allow them to keep anything back from me, lest others should be encouraged to do the like." Arnim, already master of Rügen, seized Dänholm, a smaller island commanding the mouth of the harbour. In February hostilities were commenced. In March the citizens attacked the imperialists in Dänholm, and drove them out of the island. [Sidenote: § 3. Wallenstein's first check.] It was Wallenstein's first check, and desperately did he struggle to wipe out the disgrace. Every day the spirit of the citizens was rising. There were old soldiers there, fugitives from the Danish war, and peasants who had fled from their desolated homes, and who had terrible tales to tell of the wretchedness which followed in the track of Wallenstein's soldiers. In April, all within the town bound themselves by a solemn oath to defend their religion and their liberty to the last drop of their blood, and to admit no garrison within their walls. In the midst of their resistance they still kept up some recollection of their nationality, so far as any tie of nationality could still be said to exist. The name of the Emperor was carefully avoided, but they professed attachment to the Empire and its laws. [Sidenote: § 4. Succour from Denmark and Sweden.]