It has long been argued that the intensity of violent intrastate conflicts depends crucially on the military endowments of both governments and rebel groups. However, empirical tests of this conjecture have been impossible due to the lack of appropriate data. While there are numerous measures of governments’ military capacities available, information about the military technologies of rebel groups was very limited so far. Based on a large-scale document analysis of news, media and research sources, we created a new data set for the period 1990-2018 that records the endowment of 269 non-state actors with different types of small arms, light weapons and major conventional weapons. We use this new data set to empirically test how the size and composition of rebel groups’ military arsenals, in conjunction with the government’s capabilities, affect the number of battle-related deaths in violent intrastate conflicts. The inclusion of differentiated information on rebels’ military endowments is shown to be fundamental for a better understanding of conflict dynamics.