Traveling throughout Europe, engaging in idle frivolity, and – according to his Memoirs – various love affairs, he wasted his fortune, which in 1777 forced him to transfer his estates to Henri Louis, Prince of Guéméné (grand chamberlain and captain lieutenant of the gendarmes of the king's ordinary guard), upon the payment of an annuity of 80,000 livres.
Around that time he attracted attention by an essay on the military defenses of Great Britain and her colonies (État de défense d’Angleterre et de toutes ses possessions dans les quatres parties du monde), leading to his appointment to a command against the English in 1779, in which he had some success.
Though the fortress at Cape Blanc he seized on 30 January 1779 was recaptured directly after his departure during the same year, de Vaudreuil was awarded the Grand Cross of St. Louis in 1789, whereas Lauzun received the title of colonel of hussars, and became proprietory [sic?]
[6] In 1781, he took an important part in the American War of Independence by being the advance party of the main French army of Rochambeau sent to reinforce General George Washington at the Siege of Yorktown in 1781.
[7] From the hilltop in North Stratford, now Abraham Nichols Park, one could easily see for seventy miles past Long Island Sound to New York and beyond.
On 24 October 1781, after the Siege of Yorktown, Surveillante, under Villeneuve Cillart, brought Lauzun to France to bring the news of the victory.
He several times mounted the tribune to exculpate him and in the proceedings of the Chatelet, relative to the crimes of 5 and 6 October 1789, he was accused of having appeared with him in the midst of the assassins.
In July 1792 he was named commander of the Army of the Rhine, with the duty of watching the movements of the Habsburg monarchy troops.
[3] He was accused by the notorious Jean-Baptiste Carrier of incivisme ("lack of civic virtue", the equivalent of treason[citation needed] under the Reign of Terror) and undue leniency to the insurgents, deprived of his command (July).