He had attacked, and on the 16th April, carried Landsberg, when he was apprised of the danger of Magdeburg. He resolved immediately to march to the relief of that town; and he moved with all his cavalry, and ten regiments of infantry towards the Spree. But the position which he held in Germany, made it necessary that he should not move forward without securing his rear. In traversing a country where he was surrounded by suspicious friends and dangerous enemies, and where a single premature movement might cut off his communication with his own kingdom, the utmost vigilance and caution were necessary. The Elector of Brandenburg had already opened the fortress of Custrin to the flying Imperialists, and closed the gates against their pursuers. If now Gustavus should fail in his attack upon Tilly, the Elector might again open his fortresses to the Imperialists, and the king, with an enemy both in front and rear, would be irrecoverably lost. In order to prevent this contingency, he demanded that the Elector should allow him to hold the fortresses of Custrin and Spandau, till the siege of Magdeburg should be raised. Nothing could be more reasonable than this demand. The services which Gustavus had lately rendered the Elector, by expelling the Imperialists from Brandenburg, claimed his gratitude, while the past conduct of the Swedes in Germany entitled them to confidence. But by the surrender of his fortresses, the Elector would in some measure make the King of Sweden master of his country; besides that, by such a step, he must at once break with the Emperor, and expose his States to his future vengeance. The Elector's struggle with himself was long and violent, but pusillanimity and self-interest for awhile prevailed. Unmoved by the fate of Magdeburg, cold in the cause of religion and the liberties of Germany, he saw nothing but his own danger; and this anxiety was greatly stimulated by his minister Von Schwartzenburgh, who was secretly in the pay of Austria. In the mean time, the Swedish troops approached Berlin, and the king took up his residence with the Elector. When he witnessed the timorous hesitation of that prince, he could not restrain his indignation: "My road is to Magdeburg," said he; "not for my own advantage, but for that of the Protestant religion. If no one will stand by me, I shall immediately retreat, conclude a peace with the Emperor, and return to Stockholm. I am convinced that Ferdinand will readily grant me whatever conditions I may require. But if Magdeburg is once lost, and the Emperor relieved from all fear of me, then it is for you to look to yourselves and the consequences." This timely threat, and perhaps, too, the aspect of the Swedish army, which was strong enough to obtain by force what was refused to entreaty, brought at last the Elector to his senses, and Spandau was delivered into the hands of the Swedes.