This dissertation presents one of the most comprehensive computer simulations to date of a geomagnetic storm. During the geomagnetic storm of June 4--5, 1991, five spacecraft took measurements of conditions within the inner magnetosphere, including measurements of the electric and magnetic fields and the particle distribution at several different locations. These data are used to test the theoretical understanding of magnetospheric physics by comparisons to the Rice Convection Model (RCM). The RCM is a first-principles model of the inner magnetosphere that calculates the movement of magnetospheric particles, the currents into and out of the magnetosphere, and the magnetospheric electric field patterns. Furthermore, these comparisons provide answers to some of the pressing questions in magnetospheric physics: How strong does the shielding electric field become during a magnetic storm? Can a self-consistently calculated electric field inject plasma sheet particles to within 3 Earth radii of the Earth? How much of the increase in the ring current is due to the injection of plasma sheet particles and how much is due to the energization of pre-existing particles? The comparisons between the RCM electric field results and the corresponding measurements show very good agreement between the observations and the models, and indicate that a fairly strong electric field develops during the storm. Furthermore, comparisons of RCM calculated particle fluxes and the observations indicate that a self-consistently calculated electric field is able to inject plasma sheet particles deep into the inner magnetosphere, and that this injection is the dominant cause of the increased ring current.