Problem of two emperors

The problem of two emperors or two-emperor problem (deriving from the German term Zweikaiserproblem, Greek: πρόβλημα δύο αυτοκρατόρων)^[1] is the historiographical term for the historical contradiction between the idea of the universal empire, that there was only ever one true emperor at any one given time, and the truth that there were often multiple individuals who claimed the position simultaneously. The term is primarily used in regards to medieval European history and often refers to in particular the long-lasting dispute between the Byzantine emperors in Constantinople and the Holy Roman emperors in modern-day Germany and Austria as to which monarch represented the legitimate Roman emperor.

In the view of medieval Christians, the Roman Empire was indivisible and its emperor held a somewhat hegemonic position even over Christians who did not live within the formal borders of the empire. Since the collapse of the Western Roman Empire during late antiquity, the Byzantine Empire (which represented its surviving provinces in the East) had been recognized as the legitimate Roman Empire by itself, the pope, and the various new Christian kingdoms throughout Europe. This changed in 797 when Emperor Constantine VI was deposed, blinded, and replaced as ruler by his mother, Empress Irene, whose rule was ultimately not accepted in Western Europe, the most frequently cited reason being that she was a woman. Rather than recognizing Irene, Pope Leo III proclaimed the king of the Franks, Charlemagne, as the emperor of the Romans in 800 under the concept of translatio imperii (transfer of imperial power).

Although the two empires eventually relented and recognized each other's rulers as emperors, they never explicitly recognized the other as "Roman", with the Byzantines referring to the Holy Roman emperor as the 'emperor (or king) of the Franks' and later as the 'king of Germany' and the western sources often describing the Byzantine emperor as the 'emperor of the Greeks' or the 'emperor of Constantinople'. Over the course of the centuries after Charlemagne's coronation, the dispute in regards to the imperial title was one of the most contested issues in Holy Roman–Byzantine politics. Though military action rarely resulted because of it, the dispute significantly soured diplomacy between the two empires. This lack of war was probably mostly on account of the geographical distance between the two empires. On occasion, the imperial title was claimed by neighbors of the Byzantine Empire, such as Bulgaria and Serbia, which often led to military confrontations.

After the Byzantine Empire was momentarily overthrown by the Catholic crusaders of the Fourth Crusade in 1204 and supplanted by the Latin Empire, the dispute continued even though both emperors now followed the same religious head for the first time since the dispute began. Though the Latin emperors recognized the Holy Roman emperors as the legitimate Roman emperors, they also claimed the title for themselves, which was not recognized by the Holy Roman Empire in return. Pope Innocent III eventually accepted the idea of divisio imperii (division of empire), in which imperial hegemony would be divided into West (the Holy Roman Empire) and East (the Latin Empire). Although the Latin Empire was destroyed by the resurgent Byzantine Empire under the Palaiologos dynasty in 1261, the Palaiologoi never reached the power of the pre-1204 Byzantine Empire and its emperors ignored the problem of two emperors in favor of closer diplomatic ties with the west due to a need for aid against the many enemies of their empire.

The problem of two emperors only fully resurfaced after the fall of Constantinople in 1453, after which the Ottoman sultan Mehmed II claimed the imperial dignity as Kayser-i Rûm (Caesar of the Roman Empire) and aspired to claim universal hegemony. The Ottoman sultans were recognized as emperors by the Holy Roman Empire in the 1533 Treaty of Constantinople, but the Holy Roman emperors were not recognized as emperors in turn. The Ottomans called the Holy Roman emperors by the title kıral (king) for one and a half centuries, until the Sultan Ahmed I formally recognized Emperor Rudolf II as an emperor in the Peace of Zsitvatorok in 1606, an acceptance of divisio imperii, bringing an end to the dispute between Constantinople and Western Europe. In addition to the Ottomans, the Tsardom of Russia and the later Russian Empire also claimed the Roman legacy of the Byzantine Empire, with its rulers titling themselves as tsar (deriving from "caesar") and later imperator. Their claim to the imperial title was not recognized by the Holy Roman Empire until 1745.