Rudolph, desirous to avoid a rupture, despatched his son Albert to Prague, Ottocar received him with affected demonstrations of friendship, and even bound himself by oath to fulfil the articles of the peace.  But Albert had scarcely retired from Prague before Ottocar immured in a convent the daughter he had promised to one of the sons of Rudolph, and sent a letter to the King of the Romans, filled with the most violent invectives, and charging him with a perfidious intention of renewing the war.

Rudolph returned a dignified answer to these reproaches, and prepared for the renewal of the contest which he saw was inevitable.  He instantly reoccupied that part of Austria which he had yielded to Ottocar as a pledge for the portion of his daughter.  He also obtained succors from the Archbishop of Salzburg, the bishops of Passau, Ratisbon, and the neighboring prelates and princes, and collected levies from Austria and Styria for the protection of Vienna.  In an interview at Hainburg, on the frontiers of Austria, with Ladislaus, King of Hungary, he adopted that Prince as his son, and concluded with him an offensive and defensive alliance.  Unwilling, however, to trust his hopes and fortune to his new subjects, many of whom were ready to desert him, or to allies whose fidelity and attachment were doubtful, he applied to the princes of the German empire, but had the mortification to be disappointed in his expectations.  He was joined by a few only of the inferior princes; but many who had not taken part in the former war were still less inclined to support him on the present occasion; several gained by Ottocar either remained neutral or took part against him; those who expressed an inclination to serve him delayed sending their succors, and he derived no assistance even from his sons-in-law the Electors of Palatine and Saxony.

On the other hand, he was threatened with the most imminent danger, for Ottocar, who during the peace had prepared the means of gratifying his vengeance, had formed a league with Henry of Bavaria, had purchased either the neutrality or assistance of many of the German princes, had drawn auxiliaries from the chiefs of Poland, Bulgaria, Pomerania, and Magdeburg, and from the Teutonic hordes on the shores of the Baltic.  He had also excited a party among the turbulent nobles of Hungary, and spread disaffection among his former subjects in Austria and Styria.  In June he quitted Prague, effected a junction with his allies, directing his march toward the frontiers of Austria, carried Drosendorf, after a short siege, by storm, and, descending along the banks of the Taya, invested the fortress of Laa.