Owing to the success of Protestantism, with its two forms of Lutheranism and Calvinism in various German territories, the friction became chronic between Catholic and Protestant interests throughout the length and breadth of Central Europe. The Emperor himself was chosen, as we know, by three ecclesiastical electors, the Archbishops of Koeln, Trier, and Mainz, and by four princes, the Pfalzgraf, called in English the Elector Palatine, the Markgraves of Saxony and Brandenburg, and the King of Bohemia. The princes and other potentates, owing immediate allegiance to the empire alone, were practically independent sovereigns. The Reichstag, instituted in the fifteenth century, attendance at which was strictly limited to these immediate vassals of the empire, had proved of little effect. This was shown when in the middle of the sixteenth century Protestantism had established itself in the favour of the mass of the German peoples. It was vetoed by the Reichstag, with its powerful contingent of ecclesiastical members. Of course here the economic side of the question played a great part. The ecclesiastical potentates and those favourable to them dreaded the spread of Protestantism in view of the secularization of religious domains and fiefs. This, notwithstanding that there were not wanting bishops and abbots themselves who were not indisposed, as princes of the empire, to appropriate the Church lands, of which they were the trustees, for their own personal possessions. After a short civil war an arrangement was come to at the Treaty of Passau in 1552, which was in the main ratified by the Reichstag held at Augsburg in 1555 (the so-called Peace of Augsburg); but the arrangement was artificial and proved itself untenable as a permanent instrument of peace. During the latter part of the sixteenth century two magnates of the empire, the Duke of Bavaria on the Catholic side and the Calvinist, Christian of Anhalt, on the Protestant, played the chief role, the Lutheran Markgrave of Saxony taking up a moderate position as mediator. Of the Reichstag of Augsburg it should be said that it had ignored the Calvinist section of the Protestant party altogether, only recognizing the Lutheran. In 1608 the Protestant Union, which embraced Lutherans and Calvinists alike, was founded under the leadership of Christian of Anhalt. It was most powerful in Southern Germany. This was countered immediately by the foundation under Maximilian, Duke of Bavaria, of a Catholic League. The friction, which was now becoming acute, went on increasing till the actual outbreak of the Thirty Years' War in 1618. The signal for the latter was given by the Bohemian revolution in the spring of that year.