This paper investigates the demographic consequences of militarisation in transition economies by analysing the effect of armed forces size on fertility rates across 15 post-Soviet countries from 1992 to 2022. Using panel fixed effects and two-stage least squares (FE-2SLS) with U.S. military aid as an instrument, we find that military expansion exerts a significant negative impact on fertility. Mediation analysis suggests that societal anxiety serves as a key channel. A case study of Russia highlights how sanctions and conflict further accelerate fertility decline. The findings underscore how institutional legacies of conscription shape demographic outcomes in transitional settings.