For over a century, the theory of relativity has been a secure possession of physics, and yet historians know almost nothing about its discovery, or how its proponents managed to overturn the imposing edifice of Newtonian mechanics. Recently, however, new manuscripts have come to light that inform the context of Poincaré's discovery of the Lorentz Group, i.e., the group of transformations upon which rests the special theory of relativity. In particular, these manuscripts reveal the strong influence exerted on Poincaré's thought by the work of his former student, the physicist Paul Langevin. Informed by these new archival finds, the goal is to retrace Poincaré's discovery and interpretation of the Lorentz Group, and relate this contribution to the rise of the Einstein-Minkowski theory championed by Langevin and others in France.