What led to the decline of the roman style legion?

by Tortegamos
Iguana_on_a_stick

What exactly do you mean by "Roman style legion?"

Do you mean the archetypical legion as depicted in films and such, i.e. soldiers in banded metal armour with square shields and short swords?

Or do you mean the Roman legions in general, i.e. a professional force of heavy infantry soldiers forming the core of the Roman army?