On 1 January 1771 he entered the School of Engineering at
Mézières as a second lieutenant. He was a military engineer at the
Fort de Joux in 1786, in 1789 he was captain with the Royal corps of Engineers. After the session of the National Constituent Assembly, he resumed as a captain in the engineering corps, and continued to defend the constitutional principles. On 1 January 1792,
Louis XVI gave him the
Cross of St. Louis. In 1792, he was a subordinate under
Gilbert du Motier, marquis de Lafayette, at Metz. He, Lafayette, and some other officers were taken prisoner by the Austrians near
Rochefort when de Pusy asked for rights of transit through Austrian territory on behalf of a group of French officers. This was initially granted, as it had been for others fleeing France, but was revoked when the famous Lafayette was recognized. He was imprisoned by the Austrians at the fortress of
Olomouc in 1792. He was released in 1797, under the terms of the treaty of
Campo-Formio (18 October 1797). He the then went to
Hamburg. From there he traveled to the United States, where he received a warm welcome as Lafayette companion in misfortune. He was offered vast, land grants on the banks of the
Delaware River, but he had not given up on returning to France, and when the
consulate government had, after the
coup of 18 Brumaire, struck off the list of
émigrés members of the National Constituent Assembly who had recognized the sovereignty of the people, he hastened to return. In 1799, he corresponded with
Thomas Jefferson. ==Consulate and empire==