Defence establishments in the European Union are being required to change a class of fire-protection technology that sits below the level normally associated with military capability, yet can determine whether aircraft, fuel infrastructure and industrial assets remain acceptably protected in service. The transition away from firefighting foams containing per- and polyfluoroalkyl substances is therefore not adequately described as a chemical substitution exercise. In an existing installation, the extinguishing agent is only one part of a system comprising tanks, proportioners, pumps, valves, seals, pipework, discharge devices, drainage and containment arrangements, maintenance procedures, waste routes and trained personnel. Changing the agent can alter several of those interfaces at the same time, while legacy PFAS retained on wetted surfaces can contaminate the replacement product. Legal deadlines differ by substance and by use, so the same site may face several of them at once. For defence operators, the relevant question is consequently not whether fluorine-free concentrates can be bought, but whether an existing protection system can be converted, demonstrated to remain effective, accepted for service and kept available while the transition is executed, and where along that chain the binding constraint actually lies.