The behavior of an energetic solid subjected to shear is modelled. Data from torsional split-Hopkinson bar (TSHB) experiments are first reduced to calibrate a constitutive model for stress, accounting for the effects of strain and strain rate hardening, in an inert simulant, Mock 900-20, of the high explosive LX-14. This stress model is employed in a transient one dimensional model which includes effects of thermal softening, thermal diffusion, plastic heating, and exothermic reaction. For an event with an average strain rate of 2.8 x 10(3) s(-1), reactive shear localization with local strain rates and temperatures in excess of 5 x 10(4) s(-1) and 5000 K, respectively, is predicted. It is predicted that within a domain of length comparable to that of the largest heterogeneities, around 250 mu m, reaction evolves over sub-nanosecond times scales. Localization and ignition are found to be sensitive to changes in mechanical properties, specific heat, and activation energy, and insensitive to changes in kinetic rate constant, heat release, and thermal conductivity.