The rich archbishopric, of which Magdeburg was the capital, had long been in the possession of princes of the house of Brandenburg, who introduced the Protestant religion into the province. Christian William, the last administrator, had, by his alliance with Denmark, incurred the ban of the empire, on which account the chapter, to avoid the Emperor's displeasure, had formally deposed him. In his place they had elected Prince John Augustus, the second son of the Elector of Saxony, whom the Emperor rejected, in order to confer the archbishopric on his son Leopold. The Elector of Saxony complained ineffectually to the imperial court; but Christian William of Brandenburg took more active measures. Relying on the attachment of the magistracy and inhabitants of Brandenburg, and excited by chimerical hopes, he thought himself able to surmount all the obstacles which the vote of the chapter, the competition of two powerful rivals, and the Edict of Restitution opposed to his restoration. He went to Sweden, and, by the promise of a diversion in Germany, sought to obtain assistance from Gustavus. He was dismissed by that monarch not without hopes of effectual protection, but with the advice to act with caution. Scarcely had Christian William been informed of the landing of his protector in Pomerania, than he entered Magdeburg in disguise. Appearing suddenly in the town council, he reminded the magistrates of the ravages which both town and country had suffered from the imperial troops, of the pernicious designs of Ferdinand, and the danger of the Protestant church. He then informed them that the moment of deliverance was at hand, and that Gustavus Adolphus offered them his alliance and assistance. Magdeburg, one of the most flourishing towns in Germany, enjoyed under the government of its magistrates a republican freedom, which inspired its citizens with a brave heroism. Of this they had already given proofs, in the bold defence of their rights against Wallenstein, who, tempted by their wealth, made on them the most extravagant demands. Their territory had been given up to the fury of his troops, though Magdeburg itself had escaped his vengeance. It was not difficult, therefore, for the Administrator to gain the concurrence of men in whose minds the rememberance of these outrages was still recent. An alliance was formed between the city and the Swedish king, by which Magdeburg granted to the king a free passage through its gates and territories, with liberty of enlisting soldiers within its boundaries, and on the other hand, obtained promises of effectual protection for its religion and its privileges.