I’ve been wondering about this question and decided to make this topic my first thread on this forum. I do a lot of reading on ancient near eastern empires, specially Assyriain the new Assyrian period. When compared the Achaemenid empire was much larger than that of the Assyrian empire at its peak, and it had taken the Assyrians well over 200 years to get that size. The Archamenids largest extent was under Darius I whose reign ended in 486 BC. So in what 64 years? the Archaemenids were able to rule over an empire larger than what had taken the Assyrians well over 200 years to build and much larger than all the other near eastern dynasties (Babylon, Egypt etc). How were they able to expand so fast?