On 23 March 1793 he joined a free company as a lieutenant and, the next day, saw the unit merged with the 8th Battalion of the Haute-Garonne Volunteers with Cassagne as captain.

He served with distinction in November 1794 at the Battle of the Black Mountain[2] and subsequent capture of Sant Ferran Castle at Figueres.

After the Peace of Basel ended the war with Spain, Cassagne's unit transferred to the Army of Italy.

[1] At the Battle of Loano on 22 November 1795, Cassagne was wounded in the left thigh while attacking the Rocca Barbena position.

Ordered to pursue the Austrians after the Battle of Lonato, he was seriously wounded by a musket shot in the chest on 3 August near Lake Garda.

[2] His wounding occurred on 29 May 1799 while storming a defensive work and killing every Turkish defender, at the cost of two-thirds of his own soldiers.

[2] At the Battle of Auerstadt, the 25th Line Infantry Regiment captured two cannons from the Prussians and Cassagne was hit in the face by a bullet and had a horse killed under him.

[6] Cassagne entered Spain as part of the 2nd Corps of Observation of the Gironde under Pierre Dupont de l'Étang.

[9] On 2–3 July Dupont ordered one of Vedel's brigades to march to the city of Jaén and the raid was a success.

[13] Dupont, Vedel, Armand Samuel de Marescot, Théodore Chabert and François Marie Guillaume Legendre d'Harvesse were imprisoned by Napoleon for nine months, then released.

[2] On the second day, Marshal Claude Perrin Victor ordered the divisions of François Amable Ruffin and Eugène-Casimir Villatte to turn the Allied left flank.

The British cavalry galloped unwittingly into an unseen streambed, dismounting many troopers and throwing the regiment into disorder.

After quickly realigning their ranks, the two left squadrons charged the 27th Light, which was formed into a large square, and were driven off with considerable loss.

Manuel Lapeña assumed overall command of the expedition while Thomas Graham, 1st Baron Lynedoch led the Anglo-Portuguese division.

Alerted, Victor reinforced Cassagne with three battalions and a cavalry regiment, to make a total of 3,000 men.

[18] On 4 March Cassagne reported that the Allied column was no longer headed for Medina-Sidonia, but was on a road farther west.

[20] However, Cassagne started late in the morning and Victor attacked without him[21] and was defeated by Graham's division in the Battle of Barrosa on 5 March.

Cassagne arrived only in time to rally the beaten troops and temporarily take command of Villatte's division after that general was wounded.

[22] An Army of the South organization of 3 March 1812 showed Cassagne leading the 1st Brigade of the 2nd Division under Pierre Barrois.

[26] It is not clear that Cassagne was present at Kulm because his name is not mentioned in historian Francis Loraine Petre's account, unlike other division commanders like Jean-Baptiste Dumonceau and Armand Philippon.

[32] In early October, Napoleon worried whether he should leave a strong garrison in Dresden or abandon the city and add the troops to his army.

On 11 November, Saint-Cyr surrendered on the pledge that his soldiers would be paroled to France on the promise not to fight the Allies for the remainder of the war.

[3] After his release when the war ended, Cassagne was awarded the Order of Saint Louis and given command of the Haute-Garonne department during the Bourbon Restoration.

After the second Bourbon Restoration, Cassagne was placed in inactive status and barely escaped being lynched by Royalists in the south of France.