Bolesław succeeded as Duke of Silesia after his father, Henry II the Pious, was killed in the Battle of Legnica on 9 April 1241, fighting against the Mongol invaders led by Batu Khan. At the time, he and his immediate younger brother Mieszko were the only heirs who had reached majority. Their mother Anna helped them during the transition; some sources even call this period a regency. The Mongol forces conquered most of Silesia, but then withdrew to Hungary.