He first gained military experience in the Italian Wars, and so distinguished himself at the battle of Ceresole (1544) that Louis de Bourbon, still comte d'Enghien at that time,[1] knighted him on the battlefield.

In charge of getting a relief column to Gaspard, who was defending Saint-Quentin, he found himself trapped there, but managed to prolong the siege and only surrendered when overwhelmed by the vast enemy numbers penetrating the city through the gaps they had blasted in the walls, when any resistance would have been useless.

He succeeded in escaping the Spanish camp and rejoined the French army besieging Calais, where he acted so bravely that, according to Brantôme, Francis, Duke of Guise (who was no friend of François's) stated that all he needed to conquer a world of places were Andelot, Strozzi and [Antoine] d'Estrées.

The king called Andelot before him and, on receiving confirmation from him that he had made these discourses, flew into a rage, arrested him and had him taken to the castle of Melun, where he remained until his uncle, constable Anne de Montmorency, got him bail.

When war resumed in 1568 he crossed the river Loire, penetrated into the Saintonge, captured several towns, and fought at the battle of Jarnac, where he gathered up part of what was left of the Protestant army after its retreat from Saintes.