In 1621, at the king's request, Luynes agreed to lead an expedition against the Protestants in the southwest, even though Luynes had for a long time been opposed to the campaign. As part of the agreement, Louis appointed him Constable of France, even though Luynes had slight military ability or achievement. He was sworn in on 2 April. Luynes received the post in part by default: the obvious choice, the Duke of Lesdiguières, was a Protestant and refused to abjure. Later, after Luynes's death, Lesdiguières converted and became constable. Luynes was involved in the failure of the Siege of Montauban (17 August – 2 November), for which he received much criticism, although he had not been the field commander. The Duke died of scarlet fever in December 1621 at Château de Longueville in the midst of the campaign.