In 1797 he was promoted to capitaine d’Etat-major to the army in Italy and in October the following year became first aide de camp to André Masséna in the Armée d'Helvétie.
Masséna, recognising the young man's merit, honoured him with a particular friendship and charged him once he was recovered with the formal presentation of the Italian enemy flags captured at the Battle of Marengo on 14 June 1800 to the new First Consul of France (Napoleon Bonaparte, following a coup the previous year).
Laussat was a diplomat and administrator sent by Napoleon Bonaparte to New Orleans to oversee the transfer of Louisiana (which had been Spanish since the 1760s) from Spain to France (and, as it turned out, the subsequent sale to the United States).
Laussat gathered documents generated by the Spanish colonial administration, which included royal decrees and implementing orders, regulations on local government, and official reports.
General Victor commissioned Burthe to accompany Laussat, a civilian travelling with his wife and daughters, in an advance party to arrive ahead of the troops of the main Louisiana expedition.
Laussat initially succeeded in having Burthe dropped from the expedition, but he was reinstated through the intervention of General Victor and joined the company's ship at Rochefort.
Laussat describes Burthe as having a character marked by conceit and irascibility; although he was not lacking in intelligence, his mind was disorderly and full of revolutionary ideas involving military despotism.
Laussat reports that Burthe avoided accompanying him in any formal contact with Admiral Martin, the naval prefect at Rochefort, or with the Spanish commander.
It does seem, however, that Burthe made his own separate visits to both, presenting letters from General Victor, and giving the impression that he had secret orders of his own which he did not share with the Prefect.
In the end, Laussat was the only one of these officials to arrive: Victor, who was appointed Captain-General and Commissioner to receive the Province from the Spanish on the behalf of France, never reached Louisiana.
[4] Each refused to recognise the jurisdiction of the other, and the Spanish Governor Salcedo openly received Burthe as if he were a representative of the French government, and allowing him to inspect barracks and hospitals, an action guaranteed to alienate Laussat still further.
(She was granddaughter of Pierre Antoine Foucher de Circé and older sister of Louise Delord Sarpy, who married André's younger cousin Dominique François Burthe, who had also travelled to Louisiana as part of the advance party.)
André and Suzanne left the colony soon after these events, having secretly booked passage for New York on the American ship Pastley, which they joined on 18 January 1804, and thence sailed to France.
He was promoted to Colonel of 4th Regiment of Hussars on 1 February 1805 and Suzanne, contrary to the reputation of Creole women for being modest and domestic, gaining notoriety by donning military uniform and accompanying her husband on his campaigns.
He took part in the campaigns in Prussia and Poland: despite having been wounded at Liebstadt in January, he fought at the Battle of Friedland on 14 June 1807, leading three squadrons of the 4th Hussars.
She had remained with her husband through his posting to Spain until he arrived at Bayonne, at which point she saw the wisdom of returning to Paris to become a glittering member of society and take care of her growing family.
[14] His wife died in 1863 and is buried next to him: her tomb inscription records that she was awarded the Médaille de Sainte-Hélène, participated in the campaigns of 1805-1808 and accompanied her husband to Iéna and Auerstedt.