Having captured Frankfort-on-Oder and forced the hesitating elector of Brandenburg, George William, to grant him some assistance, Gustavus Adolphus added the Saxon army to his own, and in September 1631 he met Tilly, at the head of nearly the whole force of the League, at Breitenfeld, near Leipzig, where he gained a victory which placed North Germany entirely at his feet. So utterly had he shattered the emperor's power that he could doubtless have marched straight to Vienna; he preferred, however, to proceed through central into southern Germany, while his Saxon ally, the elector John George, recovered Silesia and Lusatia and invaded Bohemia. Würzburg and Frankfort were among the cities which opened their gates to the Swedish king as the deliverer of the Protestants; several princes sought his alliance, and, making the captured city of Mainz his headquarters, he was busily engaged for some months in resting and strengthening his army and in negotiating about the future conduct of the war. Early in 1632 he led his troops into Bavaria. In April he defeated Tilly at the crossing of the Lech, the imperialist general being mortally wounded during this fight, and then he took possession of Augsburg and of Munich. Before these events Ferdinand had realized how serious had been his mistake in dismissing Wallenstein, and after some delay his agents persuaded the great general to emerge from his retirement. The conditions, however, upon which Wallenstein consented to come to the emperor's aid were remarkably onerous, but Ferdinand had perforce to assent to them. He obtained sole command of the imperial armies, with the power of concluding treaties and of granting pardons, and he doubtless insisted on the withdrawal of the Edict of Restitution, although this is not absolutely certain; in brief, the only limits to his power were the limits to the strength of his army. Having quickly assembled this, he drove the Saxons from Bohemia, and then marched towards Franconia, with the intention of crossing swords with his only serious rival, Gustavus Adolphus, who had left Munich when he heard that this foe had taken the field. The Swedes and their allies occupied Nuremberg, while the imperialists fortified a great camp and blockaded the city. Gustavus made an attempt to storm these fortifications, but he failed to make any impression on them; he failed also in inducing Wallenstein to accept battle, and he was forced to abandon Nuremberg and to march to the protection of Saxony. Wallenstein followed, and the two armies faced each other at Lützen on the 16th of November 1632. Here the imperialists were beaten, but the victory was even more disastrous to the Protestant cause than a defeat, for the Swedish king was among the slain. The league of Heilbronn and the death of Wallenstein.