This study aims to analyze the introduction of firearms in the Jesuit reductions of Paraguay during the seventeenth century. Examination of monarchy’s sources will highlight the establishment of different intermediaries and mechanisms generated by Jesuits in order to furnish firearms to Guaraní natives. Furthermore, this chapter examines the obstacles that arose in this process of distributing firearms: the fathers of the Society of Jesus clashed, in fact, with the Spanish monarchy over the topic of firearm possession, a circumstance that conditioned the training and formation of the Guaraní militias during this period. The objective of this chapter is, therefore, to investigate the effectiveness of the defense of one border in the Spanish empire, developing a model that could be applied to other colonial spaces.