id ▁al - K il abi , ▁had ▁just ▁attempted ▁a ▁campaign ▁against ▁Fer gh ana ▁and ▁suffered ▁a ▁major ▁defeat , ▁the ▁so - called ▁" Day ▁of ▁Th irst ", ▁at ▁the ▁hands ▁of ▁the ▁T urg esh ▁Tur ks ▁and ▁the ▁S og hd ian ▁principal ities ▁of ▁Trans ox iana ▁that ▁had ▁ris en ▁up ▁against ▁Muslim ▁rule . ▁ ▁From ▁the ▁early ▁days ▁of ▁the ▁Muslim ▁con quest s , ▁Arab ▁arm ies ▁were ▁divided ▁into ▁reg iments ▁drawn ▁from ▁individual ▁tribes ▁or ▁trib al ▁conf eder ations ▁( but un ▁or ▁‘ asha ‘ ir ). ▁Despite ▁the ▁fact ▁that ▁many ▁of ▁these ▁group ings ▁were ▁recent ▁cre ations , ▁created ▁for ▁reasons ▁of ▁military ▁efficiency ▁rather ▁than ▁any ▁common ▁ancest ry , ▁they ▁soon ▁developed ▁a ▁strong ▁and ▁distinct ▁identity . ▁by ▁the ▁beginning ▁of ▁the ▁Um ay y ad ▁period , ▁this ▁system ▁progress ed ▁to ▁the ▁formation ▁of ▁ever - lar ger ▁super - group ings , ▁cul min ating ▁in ▁the ▁two ▁super - groups : ▁the ▁northern ▁Arab ▁M ud aris ▁or ▁Q ays is , ▁and ▁the ▁south ▁Arab s ▁or ▁" Y emen is " ▁( Y aman ), ▁domin ated ▁by ▁the ▁Az d ▁and ▁Rab i ' ah ▁tribes . ▁By ▁the ▁ 8 th ▁century , ▁this ▁division ▁had ▁become ▁firm ly ▁established ▁across ▁the ▁C ali ph ate ▁and ▁was ▁a ▁source ▁of ▁constant ▁internal ▁inst ability , ▁as ▁the ▁two ▁groups ▁formed ▁in ▁ess ence