He was chosen by Napoleon to succeed Pierre-Charles Villeneuve as commander of the combined Franco-Spanish fleet at Cádiz, arriving to take up his appointment just after its defeat at the Battle of Trafalgar.

On Rosily's next return to France, at the end of 1774, he rushed to visit the ports of Great Britain, Scotland and Ireland, bringing back several inventions and materials of use to the French navy, such as chain pumps.

He then served as lieutenant en pied, in 1781, on board the ship of the line Fendant, before exchanging this command for that of the frigate Cléopâtre and rallying to the bailli de Suffren's squadron at Trincomalee.

Rosily then commanded the forces in the port of Rochefort before being made vice-admiral on 22 September 1796 and carrying out various missions to Genoa, La Spezia, Boulogne and Antwerp.

It is claimed Bonaparte even offered him supreme command of the fleet carrying his forces to Egypt, but that he refused as he was unwilling to leave his young family.

Rosily knew the emperor had considered him for the Egyptian post and, hoping to be given some other command soon afterwards, he asked to be promoted to a rank within the Legion of Honour more worthy of his past services.

On 18 October 1805, Villeneuve received a letter from the naval minister Decrès informing him that Rosily had arrived in Madrid with orders to take over the Cádiz command and advising "Break out, beat the enemy, and all will be righted".

However, this break-out ended in disaster at Trafalgar and then in a storm which wrecked yet more of the fleet, and so when Rosily arrived in Cádiz after the battle he found only five French ships of the line remaining rather than the 18 he was expecting.

[a] Such damage proved irreparable and though Rosily was able to make the surviving small fleet ready for sea, he remained blockaded in Cadiz by the British for two-and-a-half years until Napoleon's invasion of Spain in 1808 and the outbreak of the Peninsular War.

Adding to Rosily's problems, the six Spanish vessels remaining in the combined fleet departed and on 9 and 10 June fired over 1,200 shot on the French ships.

In 1809, he was made comte de l'Empire and a member of the council of enquiry set up to examine the conduct of Victor Hugues, the commissioner in command of French Guiana, who was accused of surrendering without a fight.

He finally retired in 1827 and, to show how he valued his services, the king granted him the title of honorary director general of the dépôt de la marine.