A Storm of Swords
After King Joffrey's death in "A Storm of Swords", Tommen is crowned and marries Joffrey's young widow, Margaery Tyrell. Tommen is a submissive child and, as a result, does everything that is asked of him. Thus, Cersei uses him to rule as she likes, though Margaery also begins to manipulate him into resisting his mother.