Superficial information of the civil wars in Aceh, Indonesia and Sri Lanka creates the idea that both conflicts were in similar situations when they were hit by the Indian Ocean tsunami in 2004. It thus seems surprising that in the wake of the tsunami, the Free Aceh Movement and the Government of Indonesia signed a peace agreement, while the Liberation Tigers of Tamil Eelam and the Government of Sri Lanka returned to war. This thesis aims to explore what factors related to the tsunami contributed to this difference and whether rational choice theory can serve as an explanation for this difference. In order to find out, I conducted a qualitative comparative case study though the analysis of secondary documents. The results suggest that the factors that contributed to the difference can be divided into four broad themes: (1) the timing of the tsunami and thus the pre-disaster context; (2) the geographical situation and with that, the military impact; (3) the types of guerilla groups, including their abilities to rule, their access to financial capital and their strategic; (4) the role of the international community, which can be further divided into firstly, the geopolitical relevance of these countries, and secondly, internationalization, community engagement and separating the tsunami and conflict. I believe that rational choice theory explains the difference in outcome between the two conflicts very well. This theory assumes that people, given the circumstances, and in view of all the possible options, will act in line with the option that is expected to satisfy them most and minimize their losses. Applying this theory to the case studies of Aceh and Sri Lanka following the tsunami, it was appealing for the Free Aceh Movement to settle, but this was not the case for the Liberation Tigers of Tamil Eelam. As a result, the former chose to sign a peace agreement with the Government of Indonesia, whereas the latter chose to continue its fight against the Government of Sri Lanka.