Why do people risk their lives fighting in wars? This article looks beyond group grievance and material benefits to add another psychological mechanism explaining why people choose to fight or not to fight – perceived collective action. An individual is much more likely to fight when they perceive that others will also fight. Contrary to the expectations of social identity theory and social pressure theory, the effect of perceived collective action is stronger among those who have a weaker national identity because they are more likely to rationally calculate the chance of winning by accounting for others’ decisions. To mitigate the endogeneity in post-conflict cross-sectional surveys, we conduct a survey experiment (n = 1,001) in Taiwan manipulating perceptions of others’ willingness to fight in a potential China–Taiwan military conflict. Experimental evidence supports the hypotheses that perceived collective action works only on weak Taiwanese identifiers. The result holds in robustness checks and in another nationally representative survey.