Otto was now again chosen German king, and to aid and mark the general reconciliation he was betrothed to the murdered king's daughter Beatrix. Nearly all the princes acknowledged him, and as pope and king were at peace, Germany enjoyed a period of comparative quiet. This however, did not last long. Having secured his coronation at Rome in October 1209, Otto repudiated the many pledges he had made to Innocent and began to act in defiance of the papal wishes. To punish him the pope put forward his own ward, Henry VI.'s son Frederick, who was living in Sicily, as a rival king. While Otto was warring in Italy a number of influential princes met at Nuremberg, at the instigation of Innocent and of his ally Philip Augustus of France, and invited Frederick to come to Germany. Otto then left Italy hurriedly, but he was quickly followed by his young rival, who in the warfare which had already broken out proved himself a formidable opponent. Seeking to mend his failing fortunes, the Welf went to France to support his ally, the English king John, against Philip Augustus, and at the battle of Bouvines (July 27, 1214) memorable in the history alike of Germany, of England and of France, his fate was sealed, although until his death in May 1218 he maintained a desultory warfare against Frederick.
Frederick II. was, if not the strongest, certainly the most brilliant of German kings. With the medieval passion for adventure he combined the intellectual culture and freedom of a modern gentleman. A lover of poetry, of art and of science, he was also a great statesman; he knew how to adapt his policy to changing circumstances and how to move men by appealing at one time to their selfishness and weakness and at another time to the nobler qualities of human nature. For outward splendour his position was never surpassed, and before he died he possessed six crowns, those of the Empire, Germany, Sicily, Lombardy, Burgundy and Jerusalem. But Germany profited neither by his gifts nor by his prestige. After Bouvines he purchased the assistance of Valdemar II., king of Denmark, by ceding to him a large stretch of land along the Baltic coast; and, promising to go on crusade, he secured his coronation at Aix-la-Chapelle in July 1215. Then being generally recognized as king he was able to do something to quell disturbances in various parts of the country, and, in April 1220, to bring about the election of his young son Henry as king of the Romans. But for this favour he had been compelled to pay a high price. Seven years before, at Eger in July 1213, he had made extensive concessions to the church, undertaking to take no part in episcopal elections, thus surrendering the advantages gained by the concordat of Worms, and to allow to German bishops the right of appeal to Rome. Proceeding a step farther in the same direction, he now promised to erect no new toll-centre, or mint, on the lands of the spiritual princes, and to allow no towns to be built thereon. Thus the prelates possessed nearly all the rights of sovereigns, and regarded the pope in Italy and not the king in Germany as their head, a state of affairs which was fatal to the unity, nay, even to the existence of the Empire.