On 11 December 1768 he was made a sous-aide major d'infanterie and sent to Corsica. He was reassigned by a royal ordnance on 1 March 1771 but even so continued to act as a sous-aide major. In a letter of 17 April 1771 Louis XV kept him in that role in recognition of his services. Later in 1771 he was made a captain in the garrison of Lorraine and on 25 July that year he left France as a captain in the force commanded by the baron de Viomesnil and M. de Choisy, sent to support the Bar Confederation in its revolt against Stanisław August Poniatowski and Russia. He captured Kraków Castle on 2 February 1772 and then defended it against attempts to re-take it. The capture was the subject of a pamphlet and made Choisy comment "I give you all my glory, my dear Saillans.". However, the Russian forces eventually proved too much for the Confederation, which had to surrender, leading to the First Partition of Poland between Russia, Austria and Prussia. After the Partition, Saillans remained in Russia as a prisoner of war until August 1773, though he was granted the cross of the Order of Saint Louis on 1 June 1772.