This article re-examines a crucial aspect of French history between 1789 and 1793, and one which remains controversial : the attitude of Louis XVI towards the Revolution. It does this by exploiting an important and unpublished source, the letters of the king's secret plenipotentiary to the European powers, the baron de Breteuil, to the foreign monarch most trusted by the French royal family, Gustavus III of Sweden. Since Louis XVI's precarious position in Paris from the October Days until his death prevented him from expressing his true feelings except very rarely, historians since have found it difficult to reach firm conclusions on his political views and motivation during the Revolution, and the result has often been partisan judgements from left and right. The issue has been further clouded by persistent claims for over a century that several of Louis's most important letters of this period are forgeries. While they do not resolve all these problems, the letters of Breteuil to Gustavus III, which are incontestably genuine, reveal Louis XVI's views on critical events between 1791 and 1792 as represented by the politician closest to his real policy, to the fellow-ruler in whom he had the most faith. The most important subjects covered are Breteuil's interpretation of Louis XVI's true attitude to the constitution of September 1791, his distrust of his brothers, the comtes de Provence and d'Artois, and the plan for an armed congress of the European powers to put pressure on revolutionary France. These letters, and Gustavus III's replies to them, are published at the end of the article in an appendix.