In response to the conflict in eastern Ukraine that began in April 2014, thousands of Ukrainians voluntarily enrolled to various paramilitary battalions. Except the Right Sector’s Volunteer Ukrainian Corps, all battalions have been incorporated into official defense and security structures. This study investigates why some combatants choose to serve in the paramilitary Right Sector’s Volunteer Ukrainian Corps rather than join the state-sanctioned military, and specifically how this choice is influenced by uncertainty. This multisite case study is conducted with volunteer fighters in the regions of Odesa and Donetsk, using data collected through interviews, observations and through the review of documents. The study applies uncertainty-identity theory, which explores how uncertainty in an individual’s own life, or their environment can prompt them to seek structure through belonging to a group or ascribing to an ideology. This theory is applied to understand how participants’ identification with the paramilitary Volunteer Ukrainian Corps might influence their feelings of self-uncertainty. The data led to six themes: Russian aggression; patriots and opportunists; enemies among us; trust, confidence and cohesion; sworn brothers; and construction of uncertainty. These themes help to understand the motivations of participants, but also how uncertainty is constructed through the membership in the Right Sector’ s Volunteer Ukrainian Corps. The findings of this study give a holistic account about participants’ feelings of self-uncertainty through their multifaceted and complex experiences. Analysis of the data revealed that the clear group prototype and the high entitativity of the Right Sector’s Volunteer Ukrainian Corps increased participants’ self-confidence, which inspired their continued identification with the group.