Sagittarii
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 major defeat in the Battle of Carrhae . 
<triplet> Crassus <subj> Battle of Carrhae <obj> conflict <triplet> Battle of Carrhae <subj> Parthians <obj> participant <subj> Crassus <obj> participant