In recent decades, negotiated settlements between governments and rebel groups have become an increasingly common solution to civil wars. Yet negotiating agreements that can endure is difficult, and a number of countries have reached settlements only to see persistent or recurrent conflict. What are the threats or obstacles to peace in countries emerging from armed conflict? I study the case of Colombia, where a 2016 peace agreement ended a decades-long conflict with the FARC, Colombia's largest and oldest rebel group. Early on, the peace agreement was lauded as an enormous success, and won the country's president a Nobel Peace Prize. Yet within only a few years of the agreement's ratification, dissident rebel factions that rejected peace had recaptured roughly half of their former territory, bringing a new wave of violence and instability. My dissertation is composed of three papers that address: (1) the causes of the rebel defection and origins of the FARC dissident factions; (2) the direct effects of the FARC dissident's defection on the peace agreement's implementation, and in particular the Disarmament, Demobilization, and Reintegration (DDR) program; and (3) the political effects of these setbacks on Colombian public's support for the peace agreement.In the first chapter, I investigate the origins of conflict resurgence: why did FARC factions defect from the peace agreement and return to war? Drawing on literatures on rebel fragmentation, peace process spoilers, and material explanations for rebellion, I argue that these dissident commanders returned to arms to exploit opportunities to profit from drug production and trafficking that, ironically, were intensified by the partial success of the peace agreement. I show several lines of evidence for this argument. Among areas previously controlled by the FARC, those with cocaine production prior to the peace agreement were significantly more likely to see dissident factions emerge by 2020 than areas without significant production. Using soil and weather conditions to instrument for cocaine production 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. Finally, I theorize and find that competition over resources from rival armed groups weakens the relationship between cocaine production capacity and FARC resurgence. These results highlight an important challenge for peacebuilders: in conflicts characterized by resource competition, demobilizing a rebel group may have the unintended consequence of increasing the opportunities for profit for the group's competitors and defectors. In Chapter 2, I focus on the consequences of the FARC dissidents' defection on the peace agreement's implementation, and specifically, the threat it poses to Colombia's Disarmament, Demobilization, and Reintegration (DDR) program. Since 2016, the former FARC combatants who demobilized have experienced a wave of violent attacks. Existing research on challenges to DDR typically focuses on proclivities for criminality among former combatants or stigmatization by civilians. I document a distinct challenge to DDR that I argue emerges when rebel groups factionalize over the decision to demobilize; namely, fratricide by rebel splinter groups that reject peace. I argue that the success of DDR threatens the interests of rebel splinter groups, and that violence against DDR participants is a strategic response to this threat. Using a difference-in-differences approach, I show that the emergence and expansion of FARC factions opposed to Colombia's 2016 peace agreement caused a surge in fatal attacks against DDR participants. I also provide qualitative evidence illustrating the mechanisms driving this pattern. These findings highlight the need for a distinct DDR model for rebel groups at risk of factionalization. Finally, Chapter 3 investigates how the defection of FARC dissidents has affected the Colombian public's support for the peace agreement. A longstanding conventional wisdom in the peacebuilding literature holds that violence during and after a peace process undermines public support for peace. Yet the empirical record is mixed, and in a few high-profile cases such as the Omagh bombing in Northern Ireland, public support for peace surged despite---or even in response to---incidents of violence. Building on the literature on public opinion formation, I argue that the effect of violence on attitudes towards peace may be moderated or exacerbated by political messaging about who or what is to blame. In Colombia, the peace agreement's political supporters and opponents offer competing messages that blame postconflict violence on either the government's implementation failures or on noncompliance by dissident rebel commanders. I fielded a survey experiment with 1466 respondents in conflict and non-conflict zones, pairing recent news about postconflict violence with information supporting these competing political messages. I find that messaging blaming rebel commanders for failing to comply reduced respondents' support for future peace negotiations, but I do not find strong evidence that blaming poor government implementation had a countervailing effect. While the treatment emphasizing rebel commanders' noncompliance increased perceptions that rebels alone were to blame, citizens responded to the treatment emphasizing government implementation failures by blaming both parties, limiting the moderating effect of this message. These results suggest that political messaging during episodes of postconflict violence can influence what citizens learn from these episodes about the viability of peace processes, but that there may be an asymmetry in citizens' propensity to assign blame that favors political opponents of peace.