Question: what was the decisive for Crassus's disastrous defeat in the most crushing defeats in Roman history ?

Related documents: [0]  [DOC] Sagittarii (Latin, plural form of "Sagittarius") is the Latin term for archers. The term "sagittariorum" in the title of an infantry or cavalry unit indicated a specialized archer regiment. Regular auxiliary units of foot and horse archers appeared in the Roman army during the early empire. During the Principate roughly two thirds of all archers were on foot and one third were horse archers. Mercenary foot archers already served with the Roman republican army, but horse archers were only introduced after the Romans came into conflict with eastern armies that relied heavily on mounted archery in the 1st century BC. Most notably the Parthians, whose mounted archers were decisive for Crassus's disastrous defeat in the Battle of Carrhae. In the early 1st century CE horse archers were already in widespread use and even supported Roman campaigns against the Germanic tribes in northern Europe. [END] 
[1]  [DOC] The Battle of Carrhae ] was fought in 53 BC between the Roman Republic and the Parthian Empire near the town of Carrhae. The Parthian Spahbod ("General") Surena decisively defeated a numerically superior Roman invasion force under the command of Marcus Licinius Crassus. 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. [END]  
Answer: mounted archers
mounted archers