Background:Following the full-scale Russian invasion of Ukraine in 2022, a large-scale training initiative in Tactical Combat Casualty Care (TCCC) was implemented with support from Swedish authorities. The effort aimed to rapidly increase lifesaving medical capacity in a war environment characterized by resource limitations, evolving threats, and extensive civilian involvement. Objective:To analyze experiences and lessons learned from the mass training of combat medics in Ukraine, with the goal of informing future preparedness and training strategies, particularly for Swedish total defense systems. Results:The training initiative successfully educated over 60,000 individuals through a train-the-trainer model. Key findings highlight the importance of instructor training as a multiplier effect, adaptation of TCCC to civilian learners, and prioritization of basic lifesaving interventions such as hemorrhage control. Challenges included mismatch between training design and wartime realities, prolonged evacuation times, resource scarcity, language barriers, and complex injury patterns. Practical, scenario-based training proved essential, while excessive reliance on standardized equipment without contextual adaptation limited effectiveness. Instructor competence, flexibility, and psychological resilience emerged as critical success factors. Conclusion:Mass training in combat casualty care during active conflict is feasible and impactful but requires substantial adaptation to context. Emphasis should be placed on instructor development, simplified training for non-experts, and practical skills. These findings provide actionable insights for strengthening national preparedness systems, particularly by initiating training early, expanding civilian capability, and integrating tactical medical knowledge into broader healthcare education.