Many studies argue that third-party guarantees, such as those of the United Nations, increase the chances that belligerents will sign peace agreements, but it is unclear how third-party involvement affects the implementation of such agreements. We unpack the relationship between UN involvement and peace agreement success by focusing on the risk factors of defections during the peace accord implementation phase. We argue that two types of commitment problems, namely involuntary and voluntary defections, emerge due to the characteristics of the peace process as well as new opportunities available to rebel groups. We expect that shifts in relative power and polarized voting lead to overall lower implementation score, but UN deployment has a mitigating effect, thereby increasing the levels of overall accord implementation. Using data from the Peace Accords Matrix Implementation Dataset from 1989 to 2010 and personnel commitments to United Nations peacekeeping operations, we find evidence that large UN missions are better placed to support the implementation and longevity of the peace process.

External supporters have heterogeneous preferences over civil war outcomes depending on the issues at stake. In this article, the bargaining model and empirical study show that such preferences of external supporters need to be considered not only when analyzing the causes of support but also when analyzing how they affect a conflict. By adding an external supporter to a traditional conflict bargaining model as a strategic actor who receives a payoff from the political division, this article investigates how the preferences of external supporters influence the likelihood of a peace agreement in civil wars over a government. I demonstrate that a peace agreement is more likely to happen when the external supporters of the government side are not satisfied with the current political status quo of the supported state. The empirical analysis of political civil wars from 1976 to 2009 supports the implications of the bargaining model.

How does the withdrawal of troops after a military intervention supporting the government affect the number of rebel groups in the long term? This study argues that the withdrawal of foreign support for the government affects the number of rebels by directly provoking a nationalist backlash in the short term and threatening government legitimacy in the long term. Whether or not nationalism is provoked and whether legitimacy is enhanced or eroded depend on whether or not it was a humanitarian intervention. If rebels win, the intervention withdrawals also indirectly affect the number of rebel groups in the long term through the militias’ presence. Using interrupted time-series estimates between 1961 and 2005, this study found that humanitarian intervention withdrawals decrease the number of rebel groups in the long term, whereas nonhumanitarian intervention withdrawals promote the growth of militias and increase the number of rebel groups.

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