▁Mik h ail ▁Ch ern y ay ev ▁( nick named ▁the ▁' Russ ian ▁General ▁C uster '), ▁and ▁was ▁assigned ▁as ▁a ide - de - camp ▁to ▁the ▁ts are v ich ▁Alexander ▁Alexand rov ich ▁( the ▁future ▁Ts ar ▁Alexander ▁III ) ▁and ▁the ▁Field ▁Marsh al ▁Prince ▁Aleks andr ▁B ary atin sky , ▁both ▁of ▁whom ▁were ▁in ▁Ser bia ▁serving ▁with ▁the ▁Russian ▁volunte er ▁expedition ary ▁force . ▁ ▁The ▁forces ▁of ▁Ott oman ▁Empire ▁defeated ▁the ▁Russian - back ed ▁Ser bi ans , ▁thus ▁leading ▁to ▁the ▁subsequent ▁Rus so - T urk ish ▁War , ▁ 1 8 7 7 – 1 8 7 8 . ▁During ▁this ▁war , ▁K eller ▁served ▁as ▁the ▁Chief - of - St aff ▁to ▁the ▁Russian - led ▁Bulgar ian ▁milit ia , ▁and ▁later ▁as ▁Chief - of - St aff ▁for ▁Russian ▁Major - General ▁Mik h ail ▁Sk obe lev , ▁where ▁he ▁replaced ▁the ▁wounded ▁Colonel ▁Alex ei ▁K urop at kin . ▁After ▁the ▁war , ▁K eller ▁was ▁promoted ▁to ▁colon el ▁and ▁served ▁in ▁various ▁staff ▁positions . ▁At ▁the ▁conclusion ▁of ▁the ▁war ▁in ▁ 1 8 7 9 , ▁he ▁was ▁promoted ▁to ▁colon el ▁and ▁designated ▁an ▁official ▁em iss ary ▁to ▁Constantin ople ▁for ▁the ▁negoti ations ▁which ▁determined ▁the ▁borders ▁of ▁the ▁Kingdom ▁of ▁Bulg aria . ▁ ▁After ▁his ▁return ▁to ▁Russia , ▁in ▁ 1 8 8 2 ,