In 1531 Philip was successful in accomplishing the purpose for which he had so long worked by securing the adhesion of the Protestant powers to the Schmalkaldic League, which was to protect their religious and secular interests against interference from the Emperor. The landgrave and his ally, John, Elector of Saxony, became recognized leaders of this union of German princes and cities. Philip was thoroughly convinced that the Protestant cause depended on weakening the power of the Habsburg emperors both at home and abroad.