The European Union’s fiscal and monetary framework, long criticised in both prosperity and crisis, now threatens to become the Achilles’ heel of European security. The current fiscal rules, obsessed with debt reduction, create an impossible bind. In an era of sluggish growth, governments cannot simultaneously slash debt and massively expand defence investment. This is not merely difficult—it is mathematically impossible. When finance ministries face this choice, the outcome is depressingly predictable. The spectre of rising bond yields will deter any serious investment programme. Governments might attempt to rob Peter to pay Paul, shifting resources from welfare or infrastructure to defence, but such austerity-driven strategies are political suicide in democracies already strained by economic uncertainty. The result will be half-measures that satisfy no one and leave Europe dangerously exposed. […]