From a military point of view the loss of Magdeburg was a crushing blow. The incipient movements in favour of Gustavus, which had begun to show themselves among the Protestant towns, at once ceased. No German princes except William of Hesse-Cassel and Bernhard of Saxe-Weimar joined him. As Gustavus slowly fell back down the Elbe, and entrenched himself at Werben, he must have felt that all the imperialist leaders had to do was to leave him alone, and his power would melt away of itself. But to leave things alone was just what Ferdinand and Maximilian in the flush of their anticipated victory could not do. In April peace had been signed at Cherasco between Ferdinand and France, and the Italian army of the Emperor had now crossed the Alps and reinforced Tilly. Forty thousand men followed his standard, and in the hope of quelling all opposition and ending the war at a blow, orders were sent to the marshal to procure the dismissal of the Saxon troops, and then to march against the Swedes. [Sidenote: =Invasion of Saxony by Tilly.=] But John George unexpectedly resented this interference with his independence. He refused to dismiss his troops. Tilly immediately occupied Merseburg and Leipzig and began harrying the country. The sight of his burning villages, and the invasion of his cherished independence, roused the sluggish elector at last. He sent messengers post haste to Gustavus offering his alliance and demanding his help. By one fatal blunder Ferdinand had done more to destroy his own cause, than all his foes together had hitherto succeeded in doing. He had driven Saxony over to the enemy. [Sidenote: =Alliance between Saxony and Sweden.=] It was not so much the material resources which the elector possessed, which made his friendship so important to Gustavus, as the position which he held in Germany. Drunken, sluggish, obstinate, irresolute as he was, men recognised in him a strenuous loyalty to the constitution of the Empire as it then existed, a hearty dread of revolutionary proposals, and a certain political shrewdness. It was these qualities, quite as much as his hereditary position as the leader of the Lutheran party, which had hitherto determined the attitude of the north German princes both towards Frederick and Christian of Denmark. That he should now join his forces to the Swedes meant that to him the foreigner and the invader appeared less of a revolutionary than the legal head of the Empire himself. [Sidenote: =The battle of Breitenfeld.=]