Meanwhile he met with cold support from the Protestant princes, in whose cause he had taken arms. The chief of these was the Elector of Saxony, who felt a jealousy, not unnatural, of the power and the ultimate views of the King of Sweden, and was himself ambitious to play the first part among the Protestants of Germany. Seeking to act independently, and to hold the balance between Sweden and Austria, he invited the Protestant States to a conference at Leipsic, February 6, 1613, at which it was determined to demand from the Emperor the redress of grievances, and to levy an army of 40,000 men, to give weight to their remonstrances. On the 13th of January, Gustavus had concluded an alliance with France, by the terms of which he was to maintain in Germany 30,000 men, France furnishing a subsidy of 400,000 dollars yearly, to use his best endeavours to reinstate those princes who had been expelled from their dominions by the Emperor, or the Catholic League, and to restore the empire to the condition in which it existed at the commencement of the war. Richelieu tried to bring the princes who had joined in the convention of Leipsic to accede to this alliance, but with very partial success. A few promised to support the Swedes, when opportunity should favour; but the Electors of Saxony and Brandenburg kept aloof. During these negotiations Gustavus made progress in Brandenburg. The memorable siege and destruction of Magdeburg, May 10, by Tilly, for a time cast a gloom over the Protestant cause. Gustavus has been censured, both as a man and a soldier, for suffering that well-deserving and important place to fall without risking a battle in its behalf. His defence rests upon the interposed delays, and the insincerity of the Electors, which involved him in the risk of total destruction if he advanced thus far without having his retreat secured. But even this signal misfortune proved finally serviceable to the Protestant cause. It induced Gustavus to adopt a different tone with his brother-in-law of Brandenburg, who, finding no alternative but a real union or an open rupture with Sweden, wisely chose the former. The pride of success led the Imperial generals into acts of insolence, which induced the Landgrave of Hesse Cassel, first of the German princes, to conclude a close and hearty alliance with Sweden, and left the Elector of Saxony no choice between entire dependence on the already exasperated Emperor, and an effective support of the only power that could protect him. Accordingly he formed a junction with the Swedes, and the united forces joined battle with Tilly not far from Leipsic, September 7, 1631. The opposing armies were nearly equal in strength. The stress of the conflict fell on the right wing of the Swedes, where the King commanded in person. The fiery Pappenheim led seven impetuous charges of the whole Austrian cavalry against the Swedish battalions without success, and, seven times repulsed, abandoned the field with great loss.