The Imperial Courts in the ten districts were now composed entirely of Catholics, and they were ordered to enforce the suppression of Protestant worship. Thereupon the Protestant princes and delegates from the cities met at the little town of Schmalkalden, in Thuringia, and on the 29th of March, 1531, bound themselves to unite, for the space of six years, in resisting the Imperial decree. Even Luther, much as he dreaded a religious war, could not oppose this movement. The League of Schmalkalden, as it is called, represented so much military strength, that king Ferdinand became alarmed and advised a more conciliatory course towards the Protestants. Sultan Solyman of Turkey, who had conquered all Hungary, was marching upon Vienna with an immense army, and openly boasted that he would subdue Germany. It thus became impossible for Charles V. either to suppress the Protestants at this time, or to repel the Turkish invasion without their help. He was compelled to call a new Diet, which met at Nuremberg, and in August, 1532, concluded a Religious Peace, both parties agreeing to refrain from all hostilities until a General Council of the Church should be called. Then the Protestants contributed their share of troops to the Imperial army, which soon amounted to 80,000 men, commanded by the famous general, Sebastian Schertlin, himself a Protestant. The Turks were defeated everywhere; the siege of Vienna was raised, and the whole of Hungary might have been reconquered, but for Ferdinand's unpopularity among the Catholic princes. [Sidenote: 1539. THE LEAGUE OF SCHMALKALDEN.] Other cities and smaller principalities joined the League of Schmalkalden, the power of which increased from year to year. The Religious Peace of Nuremberg greatly favored the spread of the Reformation, although it was not very strictly observed by either side. In 1534 Würtemberg, which was then held by Ferdinand of Austria, was conquered by Philip of Hesse, who reinstated the exiled Duke, Ulric. The latter became a Protestant, and thus Würtemberg was added to the League. Charles V. would certainly have interfered in this case, but he had left Germany for another nine years' absence, and was just then engaged in a war with Tunis. The reigning princes of Brandenburg and Ducal Saxony (Thuringia), who had been enemies of the Reformation, died and were succeeded by Protestant sons: in 1537 the League of Schmalkalden was renewed for ten years more, and the so-called "holy alliances," which were attempted against it by Bavaria and the Archbishops of Mayence and Salzburg, were of no avail. The Protestant faith continued to spread, not only in Germany, but also in Denmark, Sweden, Holland and England. The first of these countries even became a member of the Schmalkalden League, in 1538.