what was the decisive for Crassus's disastrous defeat in the most crushing defeats in Roman history ?
Sagittarii:  Most notably the Parthians, whose mounted archers were decisive for Crassus's disastrous defeat in the Battle of Carrhae. Battle of Carrhae:  It is commonly seen as one of the earliest and most important battles between the Roman and Parthian empires and one of the most crushing defeats in Roman history.
mounted archers