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This is part curiosity and part brainstorming for my TL Terra Nova Viridis (A Roman New World) yes shameless advertising, I know :D.

So the question is how would a professional Iron Age army stand against a 15th century European gunpowder army? Given that: the battle is inland where sea power has no effect , the countryside has ample food, and that disease is not a consideration. To start off discussion let's assume that the combatants are the Roman Legion and the Spanish Army.
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