Though King of England, William never renounced his title of Duke of Normandy (and Brittany) and as such was more powerful than the French Capetians who huddled around Paris and styled themselves kings. Eighty years later, in 1152 the influential, beautiful, and wily Eleanor divorced the Capetian Louis VII and married Henry Plantagenet, bringing as dowry "the richest province in all Christendom" (see The Lion in Winter, a great if psychoanalytically anachronistic film with a stellar cast), including not only the Aquitaine but also Perigord, Quercy, and adjacent areas.
The Plantagenets were sovereign over half of France and practical ruler of more than that. ("What's your claim to the Vexin?" "It's got my troops all over it, boy.") Indeed, through the vagaries of inheritance, lack of sons, and premature death, under Salic Law (daughters may not rule) they claimed the French throne; in 1340 Edward III proclaimed himself king of France and in 1415 Henry V invaded Picardy pressing the same claim.) They defended their claims through the usual practice - invade, conquer, and ratify by treaty. (The Albigensian Crusade was not only a rally for Holy Mother Church, but also a free opportunity to poach in the other fellow's game preserve and either conquer it or force Henry to defend it.)
The Hundred Years' 'war' really consisted of intermittent campaigns punctuated by 'peaces' that were little more than truces occasioned by army impoverishment, domestic affairs, or external alliances (such as the great Holy Land crusades). The two nations' fortunes also rose and fell with the lottery-draw of royal succession, as weak kings followed strong ones and great generals (Edward the Black Prince for the English, Du Guesclin and Jeanne d'Arc for the French) won huge swathes of territory, only to have it lost twenty years later by their incompetent successors. Indeed, history may have turned on a single arrow fired at Chalus (see below).
Throughout all this turmoil, the largely independent and uniformly shrewd lords of Perigord and Quercy frequently swapped sides for tactical advantage. Villagers, never knowing who might be attacking and from what direction, built fortified towns (see Bastides) and fortified their churches, leaving us an incongruous juxtaposition of crucifixions, frescoes, and stained glass alongside arrow slits, machicolations, and dry moats.