The successful assimilation of group decision support systems (GDSS) in the Command, Control, Communications, Computers, and Intelligence (C4I) systems of military organizations is critical for making fast and accurate modern military operations decisions. This study empirically examines factors affecting military group decision support systems’ (MGDSS) assimilation and the mediating effect of structural appropriation in the Korean Army in a technology-organization-environment (TOE) framework. Two hundred and forty-eight users or former users of the Army Tactical Command Information System (ATCIS) participated in the survey and a partial least squares (PLS) analysis was used to analyze the data. The results found that institutional pressure (IP) has the strongest effect on structural appropriation, followed by IT infrastructure (ITI) and top management support (TMS). Structural appropriation plays a significant mediating part between the TOE factors and MGDSS assimilation. The results provide implications and contribute valuable insights for military organizations’ successful assimilation of MGDSS.