Complex sentence: In Great Britain the natives had mined minerals for millennia, but after the Roman conquest, the scale of the operations increased dramatically, as the Romans needed Britannia's resources, especially gold, silver, tin, and lead.
Split into simpler sentences: In Great Britain the natives had mined minerals for millennia, but when the Romans came, the scale of the operations changed dramatically. The Romans needed Britannia's resources, especially gold, silver, tin, and lead.