In this paper, we explore the individual mechanisms that explain support for military alliances during wartime. We conduct a survey experiment in seven NATO countries, presenting scenarios in which Russia invades either a NATO member state (Estonia) or a non-member state (Moldova). Relying on an Audience Cost Theory (ACT) framework, we asked respondents to evaluate their leaders’ responses to the Russian invasion and to provide a justification for their support, or lack thereof, in an open-ended question. We analyse more than 40,000 responses using text-as-data classifying methods to identify the specific mechanisms shaping public opinion on military alliances. The contribution we provide is threefold. First, we test whether support for alliances is driven by solidarity and moral reasoning, ideological postures or more instrumental arguments. Second, we assess whether ACT’s hypothesised mechanisms (i.e., inconsistency, belligerence, outcome) are indeed reflected respondents’ answers. Third, we apply novel text-analysis techniques to open-ended responses in a survey experiment, showing both the pros and cons of these methods in a rather unexplored field.