While the increasing importance of water security has been widely recognised, little is known about why and when water is likely to become the target of rebel groups in civil war. This study aims to contribute to this knowledge gap and asks the question: Why do non-state actors sometimes weaponise water? Based on bargaining theory and insights from terrorism literature, this study contends that military necessity and political costs constitute the dilemma that a rebel group faces in deciding whether or not to weaponise water. Hence, the more a rebel group is faced with a need to pressure the government, the more likely the rebel group is to target water systems when the political costs are low. Through a qualitative case study involving process tracing and structured focused comparison, this thesis explores the suggested hypothesis for the case of the FARC in the Colombian conflict by comparing two periods of peace negotiations during 1991-1992 and 1998-2002. The findings suggest that political costs are a critical factor while military necessity seems of less importance in the decision to weaponise or not to weaponise. Future research is warranted and this study suggests several directions.