Having succeeded in these two important measures, Maximilian set about the more difficult enterprise of securing for his son his succession upon the imperial throne. This was a difficult matter in the strong rivalry which then existed between the Catholics and the Protestants. With caution and conciliation, encountering and overturning innumerable obstacles, Maximilian proceeded, until having, as he supposed, a fair chance of success, he summoned the diet of electors at Ratisbon. But here new difficulties arose. The Protestants were jealous of their constantly imperiled privileges, and wished to surround them with additional safeguards. The Catholics, on the contrary, stimulated by the court of Rome, wished to withdraw the toleration already granted, and to pursue the Protestant faith with new rigor. The meeting of the diet was long and stormy, and again they were upon the point of a violent dissolution. But the wisdom, moderation and perseverance of Maximilian finally prevailed, and his success was entire. Rhodolph III. was unanimously chosen to succeed him upon the imperial throne, and was crowned at Ratisbon on the 1st of November, 1575. Poland was strictly an elective monarchy. The tumultuous nobles had established a law prohibiting the election of a successor during the lifetime of the monarch. Their last king had been the reckless, chivalrous Henry, Duke of Anjou, brother of Charles IX. of France. Charles IX. having died without issue, Henry succeeded him upon the throne of France, and abdicated the crown of the semi-barbaric wilds of Poland. The nobles were about to assemble for the election. There were many influential candidates. Maximilian was anxious to obtain the crown for his son Ernest. Much to the surprise of Maximilian, he himself was chosen king. Protestantism had gained the ascendency in Poland, and a large majority of the nobles united upon Maximilian. The electors honored both themselves and the emperor in assigning, as the reason for their choice, that the emperor had conciliated the contending factions of the Christian world, and had acquired more glory by his pacific policy than other princes had acquired in the exploits of war. There were curious conditions at that time assigned to the occupancy of the throne of Poland. The elected monarch, before receiving the crown, was required to give his pledge that he would reside two years uninterruptedly in the kingdom, and that then he would not leave without the consent of the nobles. He was also required to construct four fortresses at his own expense, and to pay all the debts of the last monarch, however heavy they might be, including the arrears of the troops. He was also to maintain a sort of guard of honor, consisting of ten thousand Polish horsemen.