After the death of his father in 1267, Casimir II, together with his full-brothers, inherited their share of his lands under the regency of their mother until 1275, when they jointly ruled.
The joint Płock-Brześć-Łęczyca forces defeated on 26 February the Silesian troops commanded by Henry III of Głogów, Bolko I of Opole and Przemko of Ścinawa at the Battle of Siewierz.
[2] For unknown reasons, shortly after Bolesław II renounced to his claims over the Seniorate, an event who was used by Władysław I to conquer Lesser Poland for himself.
In the subsequent peace treaty signed on 9 October of that year, both brothers were forced to pay homage to the Bohemian King.
[5] Jan Długosz described the event as follows: Because Casimir II died unmarried and childless, his principality was inherited by Władysław I the Elbow-high.