▁of ▁gover ning ▁the ▁Empire . ▁Se jan us , ▁however , ▁al ien ated ▁Dr us us , ▁son ▁of ▁T iber ius , ▁and ▁when ▁the ▁he ir ▁to ▁the ▁throne , ▁German icus , ▁died ▁in ▁AD ▁ 1 9 , ▁he ▁was ▁wor ried ▁that ▁Dr us us ▁would ▁become ▁the ▁new ▁em peror . ▁According ly , ▁he ▁poison ed ▁Dr us us ▁with ▁the ▁help ▁of ▁the ▁latter ' s ▁wife , ▁and ▁then ▁immediately ▁launched ▁a ▁r uth less ▁elim ination ▁program ▁against ▁all ▁compet itors , ▁persu ading ▁T iber ius ▁to ▁make ▁him ▁his ▁he ir ▁apparent . ▁He ▁almost ▁succeeded , ▁but ▁his ▁plot ▁was ▁discovered ▁and ▁revealed ▁in ▁AD ▁ 3 1 ▁and ▁he ▁was ▁subsequently ▁killed . ▁Emperor ▁T iber ius ▁used ▁for ▁this ▁purpose ▁the ▁C oh ort es ▁urb ana e ▁which ▁were ▁not ▁under ▁the ▁control ▁of ▁Se jan us . ▁ ▁In ▁AD ▁ 3 7 ▁Cal ig ula ▁became ▁em peror ▁with ▁the ▁support ▁of ▁N ae vi us ▁S utor ius ▁Mac ro , ▁Se jan us ' ▁successor ▁as ▁pre fect ▁of ▁the ▁Pra et or ian ▁Guard . ▁Under ▁Cal ig ula , ▁whose ▁reign ▁last ed ▁until ▁AD ▁ 4 1 , ▁the ▁overall ▁strength ▁of ▁the ▁Guard ▁increased ▁from ▁ 9 ▁to ▁ 1 2 ▁Pra et or ian ▁coh ort s . ▁ ▁In ▁year ▁ 4 1 , ▁it ▁was ▁dis g ust ▁and ▁host ility ▁of ▁a ▁pra