Based on original qualitative data on the mobilization of anti‐Kyiv combatants during the war in eastern Ukraine (started in 2014), this article suggests an approach for understanding the spontaneous mobilization of nonstate armed groups during contemporary military conflicts. This approach is based on the notion of career as a collective path of mobilization unfolding over time. It explains how different mobilization factors usually proposed in the literature on contemporary civil wars acquire their mobilization power. The article identifies five combatant careers leading to joining the anti‐Kyiv armed groups: evolving localists, disrupted outcasts, nationalist warriors, adventurous ideologues, and inspired sympathizers. It demonstrates that (1) mobilization requires the combination of factors such as economic incentives, ideological affinity, social connections, and conflict dynamics in a particularly sequenced way, (2) this combination is different for each career path, and (3) these factors acquire their mobilizing capacity only in the context of particular social trajectories.