The European Defence Agency has reported that the European Union’s twenty-seven Member States spent €418 billion on defence in 2025, and that €115 billion of it went on equipment procurement. Against those two magnitudes, the sums that appear in this report are small and instructive: the German federal audit of the Tiger helicopter records a procurement cost of €3.6 billion for eighty aircraft, and the United Kingdom’s major-projects data records a whole-life cost of £2,904 million for a single radar programme. European rearmament increasingly depends on buying together, yet a shared programme name, a multinational contract or a common prime contractor does not establish that participating armed forces possess the same military system. National requirements can preserve genuine military utility where geography, doctrine, regulation or threat exposure differs materially. They can also divide a nominally common platform into configurations with separate development work, software, qualification evidence, support equipment, spares, upgrade paths and maintenance permissions. Each of those divisions is priced somewhere, and almost none of them is priced in public. The procurement question is therefore not whether sovereignty should give way to standardisation, and it is not what the aggregate bill for Europe’s national variants amounts to. It is whether a sovereign deviation can remain bounded throughout the equipment life cycle, so that its operational value is retained without transferring disproportionate cost, delay or loss of substitutability to the rest of the collaborative system. The unresolved question this report addresses is where that boundary is placed, who controls it, and what happens on the day a national requirement crosses it.