The war in Ukraine has been fought with, among others, irregular armed groups since 2014—volunteers, paramilitaries, and mercenaries. Based on interviews in the Russian-controlled Donbas and with Ukrainian combatants, the contributions to this volume disclose various micro-dynamics of the mobilization, group formation, and fighting. Who were these fighters and who organized them? Russia has been increasingly employing mercenaries as a way to conduct undeclared, but ruthless wars beyond her borders. Ukraine’s formation of irregular armed groups in 2014 was a response to the army’s initially glaring inability to counter Russia’s military intervention. Most of the irregular battalions acted from the beginning under governmental orders. They have never operated autonomously, but compensated for operational weaknesses of regular armed groups. The initially high power of irregular battalions derived from state support, the capabilities of commanders, social networks, and the faculties of the fighters.