Why do rebel splinter groups emerge during peace processes, and who chooses to defect? Since Colombia's landmark peace agreement with the FARC in 2016, roughly half of the territory once controlled by the group has seen a resurgence of rebel activity by FARC splinter groups. I argue that the FARC's return to arms is a case of “middle-out fragmentation,” whereby opportunities for profit induce mid- or low-ranking rebel commanders to establish splinter groups. In Colombia, I argue that profits from the cocaine trade incentivized local-level FARC officers to defect from the peace agreement and allowed them to rapidly mobilize viable splinter groups. I offer several lines of evidence for this argument. I first construct a chronology of splinter group formation, which demonstrates that mid- and low-level commanders, rather than high-level commanders, were the key drivers of fragmentation. Second, I show that splinter groups emerged in areas where opportunities for profit were greatest. Among areas previously controlled by the FARC, those with coca cultivation prior to the peace agreement were up to thirty-seven percentage points more likely to see splinter groups emerge by 2020 than areas without significant production. Using soil and weather conditions to instrument for coca cultivation produces similar results. Further, I use a novel measure of how critical each municipality is to drug trafficking to show that areas that are theoretically most important for drug trafficking are also more likely to see FARC resurgence. I also address competing explanations related to state capacity, terrain, and popular support for the rebels. These findings highlight an important challenge to peacebuilding: satisfying the political demands of rebel leadership is a necessary but insufficient component of peace agreements in cases where opportunities for profit motivate fragmentation from the middle out.