Civil wars are long, with the median conflict lasting over ten years. Armed groups need revenue to sustain their operations long-term, but we do not understand how they set or change their mix of funding strategies. Existing theory predicts that groups with resources loot to replace lost resource rents or that taxation is ideologically motivated and relatively constant across group territory. Empirically, groups with resources also create institutions and tax, and taxation varies across space. I argue that if revenue from one source declines armed groups will dynamically substitute one funding strategy for another. I test this prediction using microlevel tax data and a negative oil-price shock in a quasi-experimental design to examine how the Islamic State in Syria adapted to declining resource rents. I find that the group levied new, administratively costly taxes as a substitute for lost oil revenue. In contrast, looting did not increase. This demonstrates how groups shift the mix of funding strategies they pursue in response to the strategic environment, and that groups have preferences between different revenue sources.