Problem of two emperors
The problem of two emperors or two-emperor problem 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.
Since the collapse of the Western Roman Empire during late antiquity, the Byzantine Empire had been recognized as the legitimate Roman Empire by itself, the pope, and the various Christian kingdoms throughout Europe. This changed in 797 when Emperor Constantine VI was deposed and replaced as ruler by his mother, Empress Irene, whose rule was not accepted in Western Europe. Pope Leo III proclaimed the king of the Franks, Charlemagne, as the emperor of the Romans in 800 under the concept of translatio imperii. Although the two empires eventually recognized each other's rulers as emperors, they never explicitly recognized the other as "Roman". Over 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. On occasion, the imperial title was claimed by neighbors of the Byzantine Empire, such as Bulgaria and Serbia, which often led to military confrontations. As the Byzantine emperors had large control over the Patriarchate of Constantinople, their rivals often declared their own patriarchates independent from it.
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, in which imperial hegemony would be divided into West and East. 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. 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 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 for one and a half centuries, until the Sultan Ahmed I formally recognized 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 and later imperator. Their claim to the imperial title and equal status was not recognized by the Holy Roman Empire until 1745 and by the Ottoman Empire until 1774. While the Holy Roman Empire dissolved in 1806, the Russian rulers continued to claim the succession of the Byzantine Empire until 1917. The Greek Plan of the 1780s was the last serious attempt of restoring the Christian Byzantine Empire as a third empire alongside Russia and the Holy Roman Empire. By the 19th century, the title "emperor" and their variations became detached from Roman Empire with the title being regularly used by different states established under the rule of European royal dynasties including Austria, Brazil, Ethiopia, France, Germany, India and Mexico with little to no reference to the Roman Empire and did not claim universal hegemony. Non-European states like in East Asia also started being referred to as "empires". The latest tsars of Bulgaria and the basileis of Greece were seen as kings rather than emperors.
Background
Political background
Following the fall of the Western Roman Empire in the 5th century, Roman civilization endured in the remaining eastern half of the Roman Empire, often termed by historians as the Byzantine Empire. As the Roman emperors had done in antiquity, the Byzantine emperors saw themselves as universal rulers. The idea was that the world contained one empire and one church and this idea survived despite the collapse of the empire's western provinces. Although the last extensive attempt at putting the theory back into practice had been Justinian I's wars of reconquest in the 6th century, which saw the return of Italy and Africa into imperial control, the idea of a great western reconquest remained a dream for Byzantine emperors for centuries.Because the empire was constantly threatened at critical frontiers to its north and east, the Byzantines were unable to focus much attention to the west and Roman control would slowly disappear in the west once more. Nevertheless, their claim to the universal empire was acknowledged by temporal and religious authorities in the west, even if this empire couldn't be physically restored. Gothic and Frankish kings in the fifth and sixth centuries acknowledged the emperor's suzerainty, as a symbolic acknowledgement of membership in the Roman Empire also enhanced their own status and granted them a position in the perceived world order of the time. As such, Byzantine emperors could still perceive the west as the western part of their empire, momentarily in barbarian hands, but still formally under their control through a system of recognition and honors bestowed on the western kings by the emperor.
A decisive geopolitical turning point in the relations between East and West was during the long reign of emperor Constantine V. Though Constantine V conducted several successful military campaigns against the enemies of his empire, his efforts were centered on the Muslims and the Bulgars, who represented immediate threats. Because of this, the defense of Italy was neglected. The main Byzantine administrative unit in Italy, the Exarchate of Ravenna, fell to the Lombards in 751, ending the Byzantine presence in northern Italy. The collapse of the Exarchate had long-standing consequences. The popes, ostensibly Byzantine vassals, realized that Byzantine support was no longer a guarantee and increasingly began relying on the major kingdom in the West, the Frankish Kingdom, for support against the Lombards. Byzantine possessions throughout Italy, such as Venice and Naples, began to raise their own militias and effectively became independent. Imperial authority ceased to be exercised in Corsica and Sardinia and religious authority in southern Italy was formally transferred by the emperors from the popes to the patriarchs of Constantinople. The Mediterranean world, interconnected since the days of Roman Empire of old, had been definitively divided into East and West.
In 797, the young emperor Constantine VI was arrested, deposed and blinded by his mother and former regent, Irene of Athens. She then governed the empire as its sole ruler, taking the title Basileus rather than the feminine form Basilissa. At the same time, the political situation in the West was rapidly changing. The Frankish Kingdom had been reorganized and revitalized under King Charlemagne. Though Irene had been on good terms with the papacy prior to her usurpation of the Byzantine throne, the act soured her relations with Pope Leo III. At the same time, Charlemagne's courtier Alcuin had suggested that the imperial throne was now vacant since a woman claimed to be emperor, perceived as a symptom of the decadence of the empire in the east. Possibly inspired by these ideas and possibly viewing the idea of a woman emperor as an abomination, Pope Leo III also began to see the imperial throne as vacant. When Charlemagne visited Rome for Christmas in 800 he was treated not as one territorial ruler among others, but as the sole legitimate monarch in Europe and on Christmas Day he was proclaimed and crowned by Pope Leo III as the Emperor of the Romans.
Rome and the idea of the Universal Empire
Though the Roman Empire is an example of a universal monarchy, the idea is not exclusive to the Romans, having been expressed in unrelated entities such as the Aztec Empire and in earlier realms such as the Persian and Assyrian Empires.Most "universal monarchs" justified their ideology and actions through the divine; proclaiming themselves as either divine themselves or as appointed on the behalf of the divine, meaning that their rule was theoretically sanctioned by heaven. By tying together religion with the empire and its ruler, obedience to the empire became the same thing as obedience to the divine. Like its predecessors, the Ancient Roman religion functioned in much the same way, conquered peoples were expected to participate in the imperial cult regardless of their faith before Roman conquest. This imperial cult was threatened by religions such as Christianity, which is one of the primary reasons for the harsh persecutions of Christians during the early centuries of the Roman Empire; the religion was a direct threat to the ideology of the regime. Although Christianity eventually became the state religion of the Roman Empire in the 4th century, the imperial ideology was far from unrecognizable after its adoption. Like the previous imperial cult, Christianity now held the empire together and though the emperors were no longer recognized as gods, the emperors had successfully established themselves as the rulers of the Christian church in the place of Christ, still uniting temporal and spiritual authority.
In the Byzantine Empire, the authority of the emperor as both the rightful temporal ruler of the Roman Empire and the head of Christianity remained unquestioned until the fall of the empire in the 15th century. The Byzantines firmly believed that their emperor was God's appointed ruler and his viceroy on Earth, that he was the Roman emperor, and as such the highest authority in the world due to his universal and exclusive emperorship. The emperor was an absolute ruler dependent on no one when exercising his power. The Emperor was adorned with an aura of holiness and was theoretically not accountable to anyone but God himself. The Emperor's power, as God's viceroy on Earth, was also theoretically unlimited. In essence, Byzantine imperial ideology was simply a Christianization of the old Roman imperial ideology, which had also been universal and absolutist.
As the Western Roman Empire collapsed and subsequent Byzantine attempts to retain the west crumbled, the church took the place of the empire in the west and by the time Western Europe emerged from the chaos endured during the 5th to 7th centuries, the pope was the chief religious authority and the Franks were the chief temporal authority. Charlemagne's coronation as Roman emperor expressed an idea different from the absolutist ideas of the emperors in the Byzantine Empire. Though the eastern emperor retained control of both the temporal empire and the spiritual church, the rise of a new empire in the west was a collaborative effort, Charlemagne's temporal power had been won through his wars, but he had received the imperial crown from the pope. Both the emperor and the pope had claims to ultimate authority in Western Europe and though they recognized the authority of each other, their "dual rule" would give rise to many controversies.