Rudolf’s alliance with the Pope made him strong enough [Sidenote: War with Ottokar.] to take active measures against Ottokar, whose refusal to recognise the election on the ground that his vote had been rejected irritated the German princes. At successive diets, in 1274 and 1275, he was summoned to justify his occupation of Austria, Styria, Carinthia, and Carniola, and on his refusal was called upon to resign these fiefs. In 1276 Rudolf collected an imperial army and advanced into Austria, where he was welcomed by a general rising of the German nobles against Slav rule. Vienna capitulated, and Ottokar, finding resistance hopeless, made peace on November 21. On condition that Bohemia and Moravia should be secured to him, he resigned the German provinces. The treaty was to be confirmed by a double marriage of his daughter to Rudolf’s son Hartmann, and of his son Wenzel to one of Rudolf’s numerous daughters. Rudolf was so confident in the results of his victory, that he hastened to disband his army. But Ottokar had no intention of carrying out the treaty of Vienna, and he succeeded in gaining over many of the chief German princes by representing the danger of allowing a strong Hapsburg power to be established on the Danube. The result was a renewal of the struggle in 1278 under widely altered conditions. The death of Gregory X. (1276) had deprived Rudolf of much of the advantage gained by his concordat with the Papacy. The Archbishops of Mainz and Köln turned against him. Lewis of Wittelsbach remained obstinately neutral. Henry of Lower Bavaria, whom Rudolf had gained over in 1276 by a politic marriage, openly supported Ottokar, who was also aided by the Ascanian margraves of Brandenburg. In place of the imposing army of 1276, the only German princes who sent active aid to Rudolf were Frederick of Hohenzollern and the Bishop of Basel. But the balance was turned in his favour by the alliance of Ladislaus IV. of Hungary and by the support of the Austrian and Styrian nobles, whom Ottokar had failed to conciliate. In a great battle on the Marchfeld, the victory was decided by a charge of the heavy-armed cavalry under Frederick of Hohenzollern, and Ottokar himself perished on the field (August 26, 1278). His death made Rudolf’s victory decisive. Otto of Brandenburg, who undertook the guardianship of the young king of Bohemia, Wenzel II., negotiated a treaty in October which renewed the stipulations of 1276 as to the cession of the Austrian provinces and the double marriage between the Hapsburg and Bohemian families. In December 1282 Rudolf formally invested his sons, Albert and Rudolf, with the imperial fiefs of Austria, Styria, and Carniola. The duchy of Carinthia was given to Meinhard, Count of Tyrol, whose daughter was married to Albert of Austria.