On the memorable day of St. Bartholomew, Rosny was in Paris, engaged in the prosecution of his studies. A known member of the Protestant Church, his life was in jeopardy: his servant and his tutor fell victims to the rage of the Papists, and he himself, obliged to quit his chambers for a safer hiding-place, and exposed to imminent dangers in traversing the streets, owed his deliverance more than once to a union of courage and coolness not very common in a youth of thirteen. After this event he, as well as his patron and friend Henry of Navarre, conformed for a time to the observances of the Roman Catholic religion; but in 1576, when Henry escaped from the thraldom in which he had been held, abjured Catholicism and placed himself at the head of a Protestant army, Rosny was the companion of his flight, and first began to carry arms in his service. His noble birth, and the favour of his master, would at once have secured him military rank, but Rosny preferred to serve as a simple volunteer, in order, as he said, to learn the art of war by its elements. At the surprise of Réde, at the siege of Villefranche, at the taking of Eause and Cahors, at the battle of Marmande, and in all the dangerous affairs in which Henry engaged, Rosny was always at his side. His good services, and the affection borne him by his master, did not, however, prevent a quarrel, which, it must be said, was provoked by his own imprudence and aggravated by his own pride. In spite of the commands of the Prince of Navarre, who had wisely prohibited the practice of referring private quarrels to the arbitrement of the sword, Rosny acted as second in a duel, in which one of the principals was desperately wounded. The Prince’s anger at the breach of discipline was exasperated by a strong personal regard for the wounded man. He sent for Sully, rebuked him in harsh terms, and said that he deserved to lose his head for what he had done. The pride of the young soldier was touched; he replied that he was neither vassal nor subject of Navarre, and would henceforth seek the service of a more grateful master. The Prince rejoined in severe terms and turned his back on him; and Rosny was quitting the court, when the Queen, who knew his value, interfered, and reconciled him with her son. Not long after he quitted Henry’s service, alleging that he had pledged his word to accompany the Duc d’Alençon, afterwards Duc d’Anjou, brother of Henry III., in his contest for the sovereignty of Flanders; where, in case of success, he was to be put in possession of the estates which had belonged to his maternal grandfather. In this campaign he gained neither honour nor profit, and soon returned to his original master. Henry received him with open arms, and, as if to prove that absence had not affected his confidence and esteem, sent him a few days after on an important mission to Paris.