In 1608, he transferred to the Swedish army where he served with distinction. In 1627 the Swedish monarch knighted Alexander Leslie, by now a full colonel, along with three of his countrymen; Patrick Ruthven, David Drummond and John Hepburn. Gustavus Adolphus had a particular affection for him, trusting him with guarding the crucial strategic garrisons in North Germany while the main Swedish army established a foothold on the Baltic shoreline and advanced slowly southwards. In 1628, Leslie was appointed governor during the Siege of Stralsund, replacing Colonel Alexander Seaton and the Scottish regiment of Donald Mackay who had been holding the town on behalf of the Danes. Leslie continued the defence of Stralsund successfully defending the town against Albrecht von Wallenstein's imperial army in what effectively constituted Sweden's entrance into the Thirty Years' War, Gustavus Adolfus also sending eight warships to help raise the siege. In 1630 he seized the island of Rügen in the name of the king of Sweden.