First World War

The outbreak of the First World War found him assigned to the headquarters of the French Army. He moved rapidly through several positions in the next year, serving on the staff of the 34th Army Corps (7 November 1914) and then the staff of the Army of the Vosges, the 7th Army, and with the Belgian Army. Assigned to the staff of the 36th Army Corps on 22 May 1915, Prioux became a squadron commander on 9 April 1917 and was then assigned to the 5th Dragoon Regiment on 18 February 1918. After serving on the staff of the 164th Infantry Division, Prioux was assigned as the chief of staff of the 52nd Infantry Division on 10 February 1919.

Interwar Period

Serving with French Army headquarters in Morocco in 1919, he attended the École Supérieure de Guerre (War College) in September 1919 and was subsequently assigned to duties as an instructor at the École Supérieure de Guerre in 1921. By January 1923, he was in charge of the school's cavalry studies and was then promoted to lieutenant-colonel on 25 December 1923. On 21 August 1925 he became the commander of the 8th Spahis Regiment, which he commanded until March 1926. While on orders to report to the 11th Cuirassiers Regiment, he took part in the French military mission to Poland in 1929, of which he became commander in 1931. On 12 August 1932 Prioux was promoted to brigadier-general.

In October 1932, Prioux was assigned as the commander of cavalry in Tunisia. By March 1936, Prioux was promoted to major-general and had been assigned as the chief of cavalry at the Ministry of War. Commanding the 7th Military Region in Besançon in February 1938,^[1] he was assigned as the inspector-general of cavalry on 1 February 1939, while still commanding the 7th Region until May 1939.

Second World War

With the start of the Second World War, Prioux was assigned as the commander of the Cavalry Corps on 2 September 1939.^[2] He led this unit into Belgium after the German invasion of France and the Low Countries, meeting and severely retarding the advance of the German XVI (motorized) Corps at the battles of Hannut and Gembloux. Following the accidental death of General Billotte, Prioux took command of the French First Army from General Georges Blanchard, who had been promoted to replace Bilotte as commander of the First Army Group, on 26 May 1940, and, with much of the First Army, was taken prisoner of war three days later. The Germans allowed him to return to Vichy France in April 1942,^[3] where he was placed in the reserves in May and promoted to the rank of général d'armée in September 1942. He later served on the staff of General Henri Giraud, and was among those named for ouster by Charles de Gaulle, the leader of the Free French.^[4]

While the Vichy regime existed, Prioux promulgated antisemitic guidance to senior leaders of the Vichy French forces^[5] and was considered an enthusiastic supporter of the Vichy regime.^[6] After the Allies landed in French north Africa and the Vichy regime collapsed, Prioux was not assigned to any position of significance in the French Army and spent the remaining ten years of his life in relative obscurity.