Understanding the military capabilities of both rebel groups and state counterinsurgents is fundamental to studying the dynamics of internal conflicts. Such capabilities are expected to influence the ability of rebel groups to sustain fighting and achieve particular outcomes, as well as the types and severity of violence in which those groups engage. The size of the military forces of each rebel group is measured at the dyad-year level. In addition to the role of external support, another factor that may contribute to regional trends is the nature of how rebel groups fight. In theory, rebel groups that are able to build and maintain larger armed forces should have a better chance of securing a favorable outcome. States often face multiple internal security threats, most of which can be handled via the existing security apparatus, without needing to undertake the costly activity of mobilizing larger armed forces.