to acquire Austria by marrying his heir, Vladislaus, to the last duke's niece Gertrude of Babenberg. That marriage came to an end after half a year with Vladislaus's death in January 1247, and in 1248 Gertrude married the Zähringen margrave Herman VI of Baden. Herman, rejected by the Austrian nobility, could not establish his rule. Wenceslaus used this as pretext to invade Austria when Herman died in 1250 — according to some sources, the estates called upon him to restore order. Wenceslaus released Přemysl Ottokar very soon and in 1251 again made him Margrave of Moravia and installed him, with the approval of the Austrian nobles, as governor of Austria. The same year Ottokar entered Austria, where the estates acclaimed him as Duke. To legitimize his position, Přemysl Ottokar married the late Duke Frederick II's sister Margaret of Babenberg, who was his senior by 30 years and the widow of the Hohenstaufen king Henry (VII) of Germany. Their marriage took place on 11 February 1252 at Hainburg. In 1253, King Wenceslaus died and Přemysl Ottokar succeeded his father as King of Bohemia. After the death of the German King Konrad IV in 1254 while his son Conradin was still a minor, Ottokar also hoped to obtain the Imperial dignity - as King of the Romans - for himself. However, his election bid was unsuccessful and Count William II of Holland, the German anti-king since 1247, was generally recognised. Building an empire Feeling threatened by Ottokar's growing regional power beyond the Leitha River, his cousin King Béla IV of Hungary challenged the young king. Béla formed a loose alliance with the Wittelsbach duke Otto II of Bavaria and tried to install his own son Stephen as Duke of Styria, which since 1192 had been ruled in personal union with Austria under the terms of the Georgenberg Pact of 1186. Papal mediation settled the conflict : the parties agreed that Ottokar would yield large parts of Styria to Béla in exchange for recognition of his right to the remainder of Austria. Subsequently, King Ottokar II led the two crusade expeditions against the pagan Old Prussians (1254–1255 and 1268). Königsberg (now Kaliningrad, Russia), founded in 1255 by the Teutonic Order, was named in his honour and later became the capital of the Duchy of Prussia. After a few years of peace the conflict with Hungary resumed: Ottokar defeated the Hungarians in July 1260