On 4 June 1257 Przemysł I died, aged only 36. With his brother's death, new horizons opened to Bolesław. He became in the undisputed sole ruler over the whole of Greater Poland. Although the posthumous son of his brother, Przemysł II, was born on 14 October of that year, Boleslaw was his guardian until he was declared an adult. The first of Bolesław's new foreign politics was his marriage in 1258 to Princess Jolenta (Helena), daughter of King Béla IV of Hungary. This union resulted in a permanent bond between Bolesław and Hungary, reflected in the assistance given in the conflict with Bohemia after the extinction of the House of Babenberg. For Bolesław, this alliance cost him the complete devastation of Greater Poland during the winter of 1267-1268 by troops of King Ottokar II of Bohemia during his return from an expedition against the Prussians. One of the stages of this war was also the trip taken by Bolesław together with Bolesław V the Chaste and Leszek II the Black in the autumn of 1273 in order to visit Władysław of Opole, an ally of the Přemyslid dynasty.