His army suffered several defeats in May and June 1793, but he rallied his troops to win a defensive victory at the Battle of Perpignan in July.

At the direction of General Charles Francois Dumouriez, de Flers established the camp of Maulde in 1792 and was badly wounded defending it.

[3] The French defeat at the Battle of Neerwinden on 18 March 1793, left de Flers commanding an isolated garrison at Breda.

The Spanish Army of Catalonia under Captain General Antonio Ricardos invaded France on 17 April with 4,500 men and routed the 400-man garrison of Saint-Laurent-de-Cerdans.

[6] On 19 May, Ricardos with 7,000 troops advanced on de Flers' camp of Mas Deu, a group of medieval-era buildings established by the Knights Templar.

[7] The powerful Bellegarde fortress guarded the Le Perthus pass at 300 metres (984 ft) altitude on the main road between Barcelona and Perpignan.

De Flers tried to relieve the garrison without success, including an attempt by 3,350 men to escort a supply convoy through the siege lines on 29 May.

He also put his men to work building field fortifications around Perpignan and drafted experienced coastal artillerists to serve the guns in his redoubts.

[10] On 17 July, de Flers with 12,000 soldiers turned back an attack by Ricardos and 15,000 Spanish troops in the Battle of Perpignan.

[11] A second authority gave de Flers credit for good tactical leadership and stated that Spanish casualties numbered 1,000.

The court condemned him to die for communicating with enemies of the state and for taking part in the Luxembourg Prison conspiracies, charges which one source called a "ridiculous pretext".