What factors contributed to Charles the Great, or Charlemagne, being able to call himself an emperor of all peoples?
If a ruler is judged on imperial results, then Charlemagne, or Charles the Great (742-814), deserved to be called, or to call himself, "emperor of all peoples," although he was actually referred to as the Father of Europe or the Emperor of Rome -- world views of the time being restricted to the known civilized regions of Europe.
The factors that contributed to Charlemagne's being labeled so grandly included his successes as a military and political leader who united Europe under his command and his "coronation" as "Holy Roman Emperor" by Pope Leo III and subsequent subordination of the Byzantine Empire -- a rival for power among Christianity to the Pope -- to his rule. During his reign as emperor, from 800 to 814, Charlegmagne was arguably the most powerful person on Earth. His success in protecting the Catholic Church against the spread of Islam, and the coronation by the Pope cemented his importance in both European and Christian history.