I’ve been wondering about this for a while now, but had little free time to do my own research. What happened to all the military knowledge the Romans had, after the Western Empire fell? Things like how to make Lorica Segmentata and Musculata armours, how to fight with both pila and gladii, or how to build those complicated, yet extremely durable roads. Was that knowledge just lost, unaffordable to implement, or did it become obsolete by, say, the 8-9th century?
And a followup question, was that knowledge retained and used in the Eastern/Byzantine Empire later on?
I would guess that the changes ran more on the scales of units/battle and the way to make war, than a disappearance of the roman knowledge. What the kingdoms needed was quite different from what the roman empire needed, the whole organisation of armies and war changed. And when the massive armies and imperial administration of war aroused again, it was centuries after the fall of Rome… So big parts of that roman knowledge may have become unnecessary, and after centuries, kind of obsolete and abandoned. It could also have been diluted among factions…
But I’m no historian, that is more a guess from what I’ve read, learned, played… Sorry for my English if it seems a “little unusual”, I had to use Google Translator for some words lol