High-quality decision processes are a key success factor in Defence Investment Prioritisation (DIP). Exploring these processes entails operating in complex decision environments with various decision-makers and stakeholders, conflicting criteria, uncertainties, and inaccurate information. In this article, we propose the use of the methodological framework of Social Multi-Criteria Evaluation (SMCE), implemented through its computer model SOCRATES, to tackle DIP decisions. SOCRATES incorporates quantitative and qualitative criteria, uncertain information, the views of multiple decision-makers and stakeholders, and sensitivity analyses, as well as an equity analysis to identify credible stakeholder coalitions. To illustrate the technical capabilities of SOCRATES for DIP decisions, we use an illustrative example described in the Appendix; whilst to discuss the additional policy insights provided by SOCRATES, we revisit a case study of trainer aircraft procurement for the Spanish Air Force Academy from the literature.