The relationship between war and state formation is a central topic in the social sciences. While scholarship on interstate war posits that conflict triggers extractive processes that build the state, research findings on the effects of intrastate war are more mixed, often suggesting that civil war inhibits extraction and induces state decay. This study, however, posits that the negative relationship between civil war and revenue extraction is not underpinned by institutional destruction but by the wartime introduction of undermining rules that structure behavior in ways that subvert taxation. To illustrate this claim, it traces the evolution of undermining rules within the customs administration at the height of the Guatemalan armed conflict. As the perceived escalation of the insurgent threat created institutional ambiguity, newly empowered political-military elites implemented alternative procedures for capturing customs revenues, which systematically undermined the state's extractive capacity. Comparing this case with one of reinforcing rules that bolster extraction, I posit that the broad or narrow nature of the rule-making coalition explains divergent paths of wartime institutional development. Overall, this study uncovers the inner workings of the counterinsurgent state and illustrates how civil war dynamics induce processes of institutional change that can have long-term effects on state performance.