Emmanuel Armand de Vignerot du Plessis de Richelieu, Duke of Aiguillon (; 31 July 17201 September 1788) was a French Royal Army officer and politician. A nephew of Armand de Vignerot du Plessis, until his father's death in 1750 he was known as the Duke of Agénois. He led a regiment in Italy during the War of the Austrian Succession and was seriously wounded at the siege of Château-Dauphin (1744). He was appointed commandant of Brittany in 1753 and defended it against British landing force at Saint-Cast (1758) in the Seven Years' War. He later served as the Secretary of State for Foreign Affairs and briefly as Secretary of State for War under King Louis XV. <!--old Britannica stuff that need to recontectualize as historians' opinion: In no circumstances had he shown any special ability. He was more fitted for intrigue than for government, and his attempts to restore the status of French diplomacy met with scant success.-->
Early life and intrigue
He was the son of Armand-Louis de Vignerot du Plessis, Duke of Aiguillon (1683–1750) and Anne-Charlotte de Crussol de Florensac and so the grandson of Hortense Mancini, and until the death of his father, he was known at court as the Duke of Agénois. He entered the army at the age of seventeen, and at the age of nineteen was made colonel of the Régiment de Brie, a command he would hold until 1748. His marriage in 1740 with Louise-Félicité de Bréhan, daughter of the Comte de Plélo, coupled with his connection with the Richelieu family, gave him an important place at court.
Upon the death of King Louis XV's mistress, Madame de Vintimille, in childbirth in 1741, the king's best friend (and the duc d'Agénois' uncle), the manipulative Duke of Richelieu, began to cast about for another candidate to fulfil his royal friend's desires, as he did not want Madame de Vintimille's older sister, Madame de Mailly, to regain the king's affections. He eventually decided upon the younger sister of both Madame de Mailly and Madame de Vintimille, Marie Anne, the widow of the marquis de La Tournelle.
At a masked ball on Shrove Tuesday, 1742, Richelieu led Marie Anne up to the king and introduced them. The beautiful marquise, however, at first rejected the royal advances. She already had a lover, the young duc d'Agénois, and was not inclined to give him up even for the king's sake. As a result, the king conspired with Richelieu, who was the duc d'Agénois' uncle, to rid himself of the young suitor. Richelieu was quite anxious to do anything to bring about a liaison between the king and Madame de La Tournelle, because he knew that Madame de Mailly did not view him in a kindly light. The result of their deliberations was that the king, in imitation of the biblical David, sent his rival to fight the Austrians in Italy during the War of the Austrian Succession. The young duke was seriously wounded at the siege of Château-Dauphin (1744). Unlike the husband of Bathsheba, however, the duc d'Agénois recovered from his injuries and returned to the court in glory.
The king was in despair, but Richelieu, who was a resourceful man, was not one to accept defeat lightly. He sent his nephew to Languedoc, where a beautiful young lady had been instructed to seduce him. This she did most effectively; letters of a very passionate nature were exchanged; the lady despatched those which she received to Richelieu, and in due course they were brought to the notice of Madame de La Tournelle, who, furious at the young duke's deceitfulness, turned her attentions to the king.
The duke was later taken prisoner in 1746 and was made a maréchal de camp in 1748. He was a member of the so-called parti devot, the faction opposed to Madame de Pompadour, to the Jansenists and to the parlement, and his hostility to the new ideas drew upon him the anger of the pamphleteers.