Complex sentence: In 1211, he granted the Burzenland to the Teutonic Knights in order to quard the southeastern border of his kingdom against the Cumans, but the knights began to establish a country independent of the King of Hungary.
Split into simpler sentences: In 1211, he granted the Burzenland to the Teutonic Knights in order to ensure the security of the southeastern borders of his kingdom against the Cumans. However, the Teutonic Knights began to establish a country independent of the King of Hungary.