This article examines the way in which mathematicians were led to contribute to ballistic studies in France during World War I. It pays special attention to the French Navy's G^avre Experiments Commission rst established in 1829, where university professor Jules Haag, military engineer Maurice Garnier and high school teacher Os ee Marcus jointly developed a new method for computing ballistic trajectories (the so{called GHM method). It highlights the di culties and successes encountered by mathematicians when they approached this military culture that already was mathematically sophisticated. It reviews brie y the history of ballistics at G^avre before the First World War to understand the bitter feeling among artillerymen serving on the front about the inadequacies of their ballistic tables. In a nal part, the technical contributions made by mathematicians, their experimental practices, and their e ort for dissiminating their results are examined. This paper focuses on the role of several tensions between civilians and military science, betwen theory and experiment, between front and rear, etc. for undertanding the value of mathematicians' contributions to the war e ort.