The renewal of scientific thinking in the first half of the 17th century, along with the use of new methods of computing by engineers on the battlefield had to impact mathematics which was taught to future officers. The Kingdom of France needed the officers to be trained in the new methods, but no college, not even Universities such as the Sorbonne, offered courses on these matters. Paris had no military Academies comparable to the Academia Real Mathemática in Madrid, except Pluvinel’s Academy, where young noblemen were trained in horse riding and fencing, but probably no or few mathematical topics.From 1613 to the middle of the 1630’s, the only available mathematical textbooks in French were written by an unknown mathematician named D.Henrion, who introduced himself as a private teacher in the prefaces and the dedications of his numerous books. In fact, many new subjects could be discovered in his treatises, sometimes for the first time, like logarithms, trigonometry, fortification and the use of the proportional compass.The starting point of our research was a basic question: what does D stand for in « D. Henrion »? This question may be of no importance to historians of mathematical thought, but it’s so simple that it had to be solved first. The search for Henrion’s first name led us to investigate his family affairs, and allowed us to better understand how he became the most important mathematical popu-larizer of his time.