Sustaining professional military service is a critical challenge for small NATO member states facing prolonged security pressures and demographic constraints. This study examines the determinants of long‑term professional military service sustainability in the Lithuanian Armed Forces, situating personnel retention within a broader defense sustainability and resilience framework. Using a quantitative research design, data were collected through a structured questionnaire administered to 207 professional military service soldiers serving in the Infantry Battalion. Hierarchical regression analysis was employed to assess the incremental contribution of structural, organizational, attitudinal, and personal factors to soldiers’ intention to remain in long‑term service. The results indicate that working environment quality provides an essential structural foundation for retention, while professional satisfaction significantly enhances commitment to continued service. Most notably, personal values and family influence emerged as the strongest predictor of long‑term retention intentions, substantially increasing the explanatory power of the model. Organizational and career‑related factors showed indirect effects when more proximal determinants were considered. The findings suggest that sustainable professional military service depends not only on favourable service conditions and organizational practices but, above all, on value alignment and family support. These results underscore the importance of integrating social sustainability considerations into defence human resource policies to strengthen long‑term force resilience.