The Achaemenids fell due to a combination of
a. A weakening army and succession problems
b. Growing intolerance of other cultures and internal rebellions
c. Attacks from central Asia and weak bureaucracy
d. Bureaucratic infighting and a discontented population
The Achaemenids fell due to a combination of "b. Growing intolerance of other cultures and internal rebellions," since this led to people feeling as though they were not being respected by their government.
