From 1914 to 1918, Paris mathematicians were highly mobilized for war. In this paper, we argue that the wartime experience of Parisian mathematicians was total, in three areas especially: military expertise, scienti c and innovation policies, and intellectual debates about the relation between science and society. We establish that Paris mathematicians' early role as the unquestioned leaders of the scienti c mobilization was anchored in a prewar context where mathematics held a high status in the sciences and society. In wartime, we focus on mathematicians' activities at the Academy of Sciences and in government and pay special attention to their contribution to sound{ranging and ballistics. We fi nd that mathematicians' roles underwent signi cant changes in the course of the conflict. We finally try to assess the effects of war on postwar images of mathematics focusing on three speci c aspects: institutional reconstruction, internationalism, and modernism. In contrast to the view made popular by members of Bourbaki's fi rst generation, we argue that there was no sharp decline in postwar mathematics in Paris due to mass casualty on the front, but rather that the war generations simply adhered to views of mathematics that were strikingly at odds with the ones Bourbaki members would promote starting in the 1930s.