France during the Crusades
Louis VIII (1223-1226)
Philip's eldest son barely outlived his father, but he managed to have an impact nevertheless. While still only a prince, Louis led French forces in the Albigensian Crusade in 1215, acquiring large chunks of southern France for the crown. In 1216 he aided the rebellious barons in England and actually invaded the country. The barons even offered him the crown, a claim he did not renounce until 1218.
He also sired four sons. Because he had led armies to victory, and because of his winning personality, he continued to enhance the prestige of the crown. As king he underwrote the invasion and conquest of Languedoc, thereby greatly adding to the royal demesne. He was returning from that campaign when he fell ill from dysentery and died.
Louis was already old enough to have a son (Louis IX) before he ever became king, but that son was still a child when Louis VIII died, and the boy was left in the care of the Queen Mother, Blanche of Castile. This redoubtable woman had a tremendous influence on the boy who would eventually be known as St. Louis.