The Black Sea Fleet's progressive destruction since 2022 — examined in detail elsewhere in this series — has prompted a question that keeps recurring in defence analysis: why do capable military forces, confronted with evidence that their tactical assumptions are wrong, continue on the same trajectory until the losses become unsustainable? The Russian Navy faced the Neptune anti-ship missile, the Magura V5 unmanned surface vessel, and the Sub Sea Baby underwater drone in succession. Each system demonstrated that existing Russian countermeasures were inadequate. The institutional response was to add boom defences and helicopter patrols rather than to fundamentally reassess.

The French army at Agincourt in 1415 had faced the English longbow at Crécy in 1346 and at Poitiers in 1356. The institutional response was to add armour and more men. On 25 October 1415, on a narrow muddy field in northern France, the consequences of that failure to adapt became irrevocable.

Agincourt repays modern operational analysis not as colourful historical background but as a case study in exactly the failure modes that produce military catastrophe: terrain misread, command dysfunction, technology underestimated, and an enemy consistently undervalued until undervaluing him was no longer possible. The rhyme across six centuries is audible. What follows is an attempt to make it precise.