Charles IX of France

Charles Maximilien became King of France at ten in 1560, the penultimate Valois monarch, inheriting decades of Catholic-Protestant tension that erupted into civil war after the massacre of Vassy in 1562. A decade of failed peace efforts led him to arrange his sister Margaret's marriage to Protestant nobleman Henry of Navarre in 1572, a last attempt at reconciliation. Facing popular backlash and urged on by his mother Catherine de' Medici, he oversaw the massacre of Huguenot leaders gathered for the wedding - the St. Bartholomew's Day massacre - though his direct role remains disputed. The killing rekindled religious war; Charles tried to capitalize by besieging the Protestant stronghold of La Rochelle but failed to take it. The massacre haunted him, and historians believe it worsened his physical and mental decline. He died of tuberculosis in 1574 at twenty-three, leaving no legitimate male heir, and his brother Henry III succeeded him.