But he abandoned theology for more secular studies, especially that of ancient history, in which his learning attracted the notice of the Prince of Orange, who promised him a vacant professorship at Utrecht.
Coming back, with English introductions, to the Continent, he sought service with Ferdinand of Brunswick, who sent him on to Frederick the Great, whom he joined in January 1758 at Breslau.
The place cannot have been seriously injured, as it was soon afterwards the meeting-place of the diplomats whose work ended in the Treaty of Hubertusburg, but the king never ceased to banter Quintus on his supposed depredations.
[1] The very day of Frederick's triumphant return from the war saw the disbanding of most of the free battalions, including that of Quintus, but the major to the end of his life remained with the king.
He was made lieutenant-colonel in 1765, and in 1773, in recognition of his work Mémoires critiques et historiques sur plusieurs points d'antiquités militaires, dealing mainly with Julius Caesar's campaigns in Spain (Berlin, 1773), was promoted colonel.