Combat-vehicle fleets operating in expeditionary, contested, or otherwise logistically isolated theatres routinely face spare-part availability shortfalls that ground a substantial share of the fleet for weeks or months while replacement components transit a vulnerable conventional supply chain. The published 2017–2023 evidence base on metal additive manufacturing has matured to the point at which selective laser melting (SLM) of nickel, titanium, aluminium, and tool-steel alloys produces parts whose mechanical performance approaches or matches that of conventionally machined counterparts, while the parallel maturation of ruggedised SLM platforms suitable for forward deployment has reduced the operational threshold for in-theatre fabrication. Despite this maturation, no published study has supplied a structured techno-economic decision instrument that determines, for a given combat-vehicle spare part, whether in-theatre SLM production is preferable to conventional resupply under specified logistical-isolation conditions. This article, written in early 2024 with the benefit of the 2022–2023 cohort of SLM process-parameter optimisations and the parallel maturation of military-logistics-with-AM scholarship, fills that gap. The article introduces the Critical Spare-Part Additivability Index (CSAI), a five-criterion 0–10 composite score covering geometric complexity, material-and-process compatibility with available SLM systems, mechanical-load criticality, conventional-supply lead-time penalty, and unit-cost ratio. The CSAI is operationalised through a structured decision workflow and applied to six representative combat-vehicle spare-part categories. Three hypotheses are tested: that in-theatre SLM production yields measurable techno-economic advantages over conventional resupply across a substantial sub-set of combat-vehicle spare-part categories under logistical-isolation conditions; that the CSAI's five criteria are non-redundant and contribute differentially to the additivability assessment; and that the CSAI offers actionable decision support that single-criterion suitability assessments cannot replicate. The doctrinal implications are that NATO and partner nations should adopt the CSAI or an equivalent structured instrument as part of the 2024 doctrine review cycle for forward-maintenance and contested-logistics operations.