For the present, however, fresh difficulties with France and an invasion by the Turks, who had besieged Vienna with an immense army in the autumn of 1529, forced Charles to mask his designs. Meanwhile some of the Lutherans, angered and alarmed by the decisions of the _Reichskammergericht_, abandoned the idea that resistance to the imperial authority was unlawful and, meeting in December 1530, laid the foundation of the important league of Schmalkalden, among the first members of the confederation being the rulers of Saxony and Hesse and the cities of Bremen and Magdeburg. The league was soon joined by other strong cities, among them Strassburg, Ulm, Constance, Lübeck and Goslar; but it was not until after the defeat and death of Zwingli at Kappel in October 1531 that it was further strengthened by the adhesion of those towns which had hitherto looked for leadership to the Swiss reformer. About this time the military forces of the league were organized, their heads being the elector of Saxony and the landgrave of Hesse. But the league had a political as well as a religious aspect. It was an alliance between the enemies of the house of Habsburg, and on this side it gained the support of the duke of Bavaria and treated with Francis I. of France. To this its rapid growth was partly due, but more perhaps to the fact that the Reformation in Germany was above all things a popular movement, and thus many princes who would not have seceded from the Roman Church of their own accord were compelled to do so from political motives. They had been strong enough to undermine the imperial power; they were not strong enough to resist the pressure put upon them by a majority of their subjects. It was early in 1532, when faced with the necessity of resisting the Turkish advance, that Charles met the diet at Regensburg. He must have men and money for this purpose even at the price of an arrangement with the Protestants. But the Lutherans were absent from the diet, and the Romanists, although they voted help, displayed a very uncompromising temper towards their religious foes. Under these circumstances the emperor took the matter into his own hands, and his negotiations with the Protestants resulted in July 1532 in the religious peace of Nuremberg, a measure which granted temporary toleration to the Lutherans and which was repeatedly confirmed in the following years. Charles's reward was substantial and immediate. His subjects vied with each other in hurrying soldiers to his standard, and in a few weeks the great Turkish host was in full retreat.