MARQUIS DE (1620-1642), French courtier, was the second son of Antoine Coiffier Ruze, marquis d'Effiat, marshal of France (1581-1632), and was introduced to the court of Louis XIII. by Richelieu, who had been a friend of his father and who hoped he would counteract the influence of the queen's favourite, Mlle. de Hautefort. He became the king's accredited favourite, master of the wardrobe and master of the horse. After distinguishing himself at the siege of Arras in 1640, Cinq-Mars became restive. He had fallen in love with Louise Marie de Gonzaga, afterwards queen of Poland, who was not disposed to favour a suitor who could not satisfy her ambition. Cinq-Mars demanded a high military command, but Richelieu opposed his pretensions and the favourite talked rashly about overthrowing the minister. He was probably connected with the abortive rising of the count of Soissons in 1641; however that may be, in the following year he formed a conspiracy with the duke of Bouillon and others to overthrow Richelieu. This plot was under the nominal leader ship of the king's brother, Gaston of Orleans. The plans of the conspirators were aided by the illness of Richelieu and his absence from the king, and at the siege of Narbonne Cinq-Mars almost in duced Louis to agree to banish his minister. Richelieu, however, recovered; Gaston betrayed the conspirators, and Richelieu laid before the king the proofs of the treasonable negotiations of the conspirators for assistance from Spain. Cinq-Mars was brought to trial, admitted his guilt, and was executed at Lyons on Sept. 1642. The conspiracy of Cinq-Mars forms the subject of a novel by Alfred de Vigny, Cinq-Mars.
See d'Hancour, La Conspiration de (1902).