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 bowmen were foot and one third horse archers. While mercenary foot archers already served with the Roman republican army, horse archers were only introduced after the Romans came into conflict with eastern factions that relied heavily on mounted archery in the 1st century BCE. Most notably the Parthians which mounted archers were decisive for Crassus disastrous defeat in the Battle of Carrhae. In the early 1st century CE horse archers were already in widesprea