Language is one of the essential skills for soldiers, particularly in peacekeeping and multilateral operations, as communication skills enhance the effectiveness of operations and foster intercultural understanding. This study investigates motivational drivers of second language (L2) learning by military staff in light of Self-Determination Theory (SDT) and the distinction between instrumental and integrative motivation. Using a mixed-methods approach, the study involved 45 Iranian military learners with quantitative questionnaires, qualitative interviews, and language proficiency tests. Findings suggest instrumental motivation, as stimulated by career advancement and mission success, to be the primary motivator of L2  learning, with intrinsic and integrative motivations as secondary motivators. Issues such as limited opportunities for language use, no immersion environments, and a balance between language training and other duties influence the long-term retention of languages. The study stresses that military training programs must incorporate immersion experiences, intrinsic and integrative motivation, and long-term opportunities for language practice. These results contribute to the literature on L2 acquisition in specialized contexts and offer practical recommendations for the enhancement of language training in military settings.