The fact that the Romans could give every legionnaire (or nearly so) a galea, lorica, and gladius was very impressive and wasn't something that could be replicated in Europe until the Renaissance. It's not just the quantity, but quality and size too. They could spin metal on lathes. Too much slag in the metal, and that wouldn't work. They could make helmets out of one piece of steel. Medieval smiths had to resort to riveting smaller pieces together until the 14th century or so. I guess what I'm asking is, how did the Romans produce so much high quality metal?
I'd like to ask a similar question:
It's not just metal, is it? One of signs of the Romans leaving Britain was the local residents starting to recycle old iron and glass. If I recall correctly, that iron, glass, and other goods must've been imported.
Was there a 'global' trading network of goods similar to how we have a global shipping system today, where it is cheaper to buy shoes made half way around the world? What factors lead to this price discrepancy? What infrastructure supported this system? And how did (and what impact did its disappearance) affect the members in this system?