Her next famous undertaking, which involved the lives of various persons of high rank, was the scheme to persuade Monsieur the Dauphin to refuse to marry Mlle. de Montpensier; Queen Anne was opposed to this union, and Mme. de Chevreuse gained to their cause a number of influential friends who were all madly in love with her.  The ever vigilant Richelieu having discovered the plot, Monsieur confessed.  In this conspiracy, M. de Chalais lost his head, other plotters lost their positions, and some were exiled.  Mme. de Chevreuse was forced to retire to Lorraine; there she set in movement a vast plan against Richelieu and France, allying England and various princes, but, by the arrest of Montaigu, the plot was discovered, the alliance broken up, and peace restored.

In 1626, by request of England, Mme. de Chevreuse returned to France.  For a time she was quiet and seemed to favor Richelieu, but she soon captivated one of his ministers, the Marquis of Chateauneuf.  Richelieu discovered the latter’s weakness, and, having captured his correspondence, sent him to prison, where he remained for ten years.  The fair intriguer was exiled to Dampierre, the cardinal fearing to send her out of France on account of her influence with the Duke of Lorraine.  She managed to steal into Paris at night and see the queen; when discovered, she was sent to Touraine where she began the dangerous task of carrying on the correspondence between the Dukes of Savoy and Lorraine and England, and between Spain and Queen Anne.  Even when this correspondence was intercepted and the queen confessed all, Richelieu was afraid to banish Mme. de Chevreuse; though he believed her to be at the bottom of all the current intrigues, he knew that out of France she would stir up the rulers of England and Spain as well as the Duke of Lorraine and others hostile to the cardinal.

Violence being out of the question, because of her influence in England and of the prominence of her family, he decided to win her over by kindness; he even sent her money, but she was too shrewd to permit Richelieu to outwit her, always paying him back in his own coin.  However, that kind of play was too dangerous for her and she escaped to Spain.  As soon as her departure became known, Richelieu set to work every means in his power to bring her back, sending her an urgent invitation to return and promising to pardon her past.  When his messages reached her, she was already in Madrid, where she was royally received as the friend of the king’s sister, Anne; there, by means of her beauty and wonderful intelligence, she conquered every cavalier.  When the war broke out between France and Spain, she left for England where she was welcomed like a visiting queen.