premise: Otto I, Holy Roman Emperor ended the Bulgarian invasions.
hypothesis: Otto I ( 23 November 912 -- 7 May 973 ) , traditionally known as Otto I the Great ( Otto I. der Große ) , was German king from 936 and emperor of the Holy Roman Empire from 962 until his death in 973 . After putting down a brief civil war among the rebellious duchies , Otto defeated the Magyars at the Battle of Lechfeld in 955 , thus ending the Hungarian invasions of Western Europe .