Short answers, pulled from the story.
Jacques-Antoine-Hippolyte, Comte de Guibert, was a French general and military writer born on the 12th of November 1743 in Montauban. He published his landmark Essai général de tactique in 1770 and later served as a member of the Académie française from 1786. He died on the 6th of May 1790.
The Essai général de tactique, published in London in 1770, argued that standing armies were an inadequate and burdensome instrument and that military dominance would fall to the nation that created a national army drawn from the broader population. The essay placed heavy emphasis on speed and movement and was judged the best military essay of its era by later scholars. It was translated into English, German, and Persian.
In the Essai général de tactique, Guibert wrote that hegemony over Europe would fall to the nation that creates a national army from a population trained in arms. Within twenty years of his death in 1790, that prediction had been fulfilled almost to the letter by France under Napoleon, who built his campaigns on precisely the emphasis on speed and national mobilization that Guibert had described.
In 1773, Guibert traveled to Germany and observed Prussian army drills and maneuvers. Frederick the Great personally recognized Guibert's ability, showed him great favor, and discussed military questions with him directly. Guibert's published account of that visit, the Journal d'un voyage en Allemagne, appeared posthumously in Paris in 1803.
Julie de Lespinasse was a celebrated figure in Parisian salon culture who fell in love with Guibert during the years he was collaborating with the Comte de Saint-Germain on French army reforms. The love letters she wrote to him were later published and remain read today.
From 1775, Guibert cooperated with the Comte de Saint-Germain on a series of reforms to the French army that the source describes as much-needed and successful. The reforms drew on Guibert's study of Prussian tactical methods. When Saint-Germain fell into disgrace in 1777, Guibert's career suffered alongside his, and he was reassigned to a provincial staff post.