Complex sentence: In 1212 the French king needed an obedient vassal to marry Alix, Duchess of Brittany and turned to his cousin Peter, a younger son of the Count of Dreux.
Split into simpler sentences: In 1212 the French king needed an obedient vassal to marry Alix, Duchess of Brittany. He turned to his cousin Peter, a younger son of the Count of Dreux.