He served subsequently as a volunteer aide on the staff of Gen.
In 1821 Alexander Ypsilanti, a son of the voivode, and an aide-de-camp of the tsar Alexander I., entered Moldavia at the head of the Hetaerists, and, representing that he had the support of the tsar, prevailed on the hospodar Michael Sutzu to aid him in invading the Ottoman dominions.
In 1795 he accompanied Bernadotte as aide-de-camp. In 1799 he was promoted chef d'escadron, and in 1800 colonel.
He became aide-de-camp to Murat, and was wounded at the battle of Landbach in 1805.
Flahaut fought with distinction in the Russian campaign of 1812, and in 1813 became general of brigade, aide-de-camp to the emperor, and, after the battle of Leipzig, general of division.