The Protestant princes, however, although in the utmost peril and humiliated to the dust, refused to unite with him. If their course had been cowardly and selfish before, it now became simply infamous. The Duke of Pomerania shut the gates of Stettin upon the Swedish army, until compelled by threats to open them; the Electors of Brandenburg and Saxony held themselves aloof, and Gustavus found himself obliged to respect their neutrality, lest they should go over to the Emperor's side! Out of all Protestant Germany there came to him a few petty princes whose lands had been seized by the Catholics, and who could only offer their swords. His own troops, however, had been seasoned in many battles; their discipline was perfect; and when the German people found that the slightest act of plunder or violence was severely punished, they were welcomed wherever they marched. [Sidenote: 1631.] Moving slowly, and with as much wisdom as caution, Gustavus relieved Pomerania from the Imperial troops, by the end of the year. He then took Frankfort-on-the-Oder by storm, and forced the Elector of Brandenburg to give him the use of Spandau as a fortress, until he should have relieved Magdeburg, the only German city which had forcibly resisted the "Edict of Restitution," and was now besieged by Tilly and Pappenheim. As the city was hard pressed, Gustavus demanded of John George, Elector of Saxony, permission to march through his territory: it was refused! Magdeburg was defended by 2,300 soldiers and 5,000 armed citizens against an army of 30,000 men, for more than a month; then, on the 10th of May, 1631, it was taken by storm, and given up to the barbarous fury of Tilly and his troops. The city sank in blood and ashes: 30,000 of the inhabitants perished by the sword, or in the flames, or crushed under falling walls, or drowned in the waters of the Elbe. Only 4,000, who had taken refuge in the Cathedral, were spared. Tilly wrote to the Emperor: "Since the fall of Troy and Jerusalem, such a victory has never been seen; and I am sincerely sorry that the ladies of your imperial family could not have been present as spectators!"