Many civil wars are illustrative of wider international tensions and connections that transcend state borders. States often intervene to influence the trajectory and outcome of civil conflicts by providing external support to warring parties. This assistance ranges from direct military intervention to the provision of weapons, training, funds, safe havens, intelligence, logistics and other critical resources. This dissertation contains four individual essays that each seeks to advance our knowledge of state support to rebel movements. The first essays (I and II) add to our understanding of how external state support influences conflict dynamics while the latter (III and IV) begin to unpack the political decision-making process behind decisions that alter the original support commitment. Essay I evaluates whether state support to rebels increases the probability of civil war negotiations being initiated. The findings question a widespread belief among policymakers that support can foster negotiations. Essay II explores if external support influences the risk of conflict recurrence. It finds that state support to rebels can increase the risk of conflict recurrence in the short-term while there is no equivalent effect of support provided to governments. Essay III is the first global analysis of support termination and it thereby opens up an entirely new research field. The results suggest that the causes related to the initiation of support and its termination are largely distinct while the transition from the Cold War and the absence of ethnic kinship ties offer some insights into when states are more likely to terminate support. Essay IV unpacks the political decision-making process of the United States’ support to the armed opposition in Nicaragua in the 1980s and in Syria in the 2010s. The results indicate that adverse feedback functions as a trigger for increasing previous commitments as long as policy failure can be attributed to external actors, while reduced support is often a result of attributing failure to the state sponsor’s own actions. Taken together, the essays make significant contributions to advance our understanding of biased third-party interventions, conflict recurrence, civil war negotiations, foreign policy decision-making and state sponsorship of terrorism.