▁Anton inus ▁P ius . ▁He ▁may ▁have ▁been ▁ad lect ed ▁as ▁a ▁qu a est or ▁in ▁ 1 5 4 ▁AD . ▁It ▁is ▁thought ▁that ▁he ▁became ▁a ▁leg atus ▁of ▁one ▁of ▁the ▁leg ions ▁station ed ▁in ▁Mo esia ▁In fer ior , ▁which ▁guard ed ▁against ▁the ▁S arm ati ans , ▁during ▁the ▁late ▁years ▁of ▁P ius ' ▁rule ▁( 1 3 8 – 1 6 1 ), ▁and ▁it ▁is ▁certain ▁that ▁he ▁was ▁a ▁leg atus ▁by ▁at ▁least ▁ 1 6 1 AD , ▁the ▁last ▁year ▁of ▁P ius ' ▁reign . ▁ ▁Cass ius ▁rose ▁to ▁prom in ence ▁rapidly ▁ 1 6 4 AD , ▁under ▁the ▁co - em per ors ▁Lu cius ▁Ver us ▁and ▁Marcus ▁A ure lius , ▁during ▁the ▁Par th ian ▁War ▁of ▁Lu cius ▁Ver us , ▁serving ▁as ▁the ▁leg atus ▁of ▁Leg io ▁III ▁Gall ica . ▁In ▁ 1 6 5 , ▁he ▁led ▁Leg io ▁III ▁Gall ica ▁down ▁the ▁Eu phr ates , ▁and ▁defeated ▁the ▁Par th ians ▁at ▁D ura - E uro pos . ▁Before ▁the ▁end ▁of ▁the ▁year , ▁Cass ius ▁and ▁his ▁leg ion ▁march ed ▁to ▁the ▁south , ▁crossed ▁Mes op ot am ia ▁at ▁its ▁narrow est ▁point , ▁and ▁attacked ▁and ▁s ack ed ▁the ▁tw in ▁Par th ian ▁cities ▁of ▁the ▁Tig ris ▁river : ▁Sele u cia , ▁which ▁was ▁on ▁the ▁right