In 1773 he visited Germany and was present at the Prussian regimental drills and army manœuvres; Frederick the Great, recognizing Guibert's ability, showed great favour to the young comte and freely discussed military questions with him. Guibert's "Journal d'un voyage en Allemagne" was published, with a memoir, by Toulongeon (Paris, 1803). His "Défense du système de guerre moderne", a reply to his many critics (Neuchâtel, 1779) is a reasoned and scientific defence of the Prussian method of tactics, which formed the basis of his work when in 1775 he began to co-operate with the count de Saint-Germain in a series of much-needed and successful reforms in the French army. During those years, he also won the love of Julie de Lespinasse, whose love letters to him, later published, are still read today.