Bulgaria
Further information: List of Bulgarian monarchs
The sainted Boris I is sometimes retrospectively referred to as tsar, because at his time Bulgaria was converted to Christianity. However, the title "tsar" (and its Byzantine Greek equivalent "basileus") was actually adopted and used for the first time by his son Simeon I, following a makeshift imperial coronation performed by the Patriarch of Constantinople in 913. After an attempt by the Byzantine Empire to revoke this major diplomatic concession and a decade of intensive warfare, the imperial title of the Bulgarian ruler was recognized by the Byzantine government in 924 and again at the formal conclusion of peace in 927. Since in Byzantine political theory there was place for only two emperors, Eastern and Western (as in the Late Roman Empire), the Bulgarian ruler was crowned basileus as "a spiritual son" of the Byzantian basileus.
Some of the earliest attested occurrences of the contraction "tsar" (car' ) from "tsesar" (cěsar' ) are found in the grave inscription of the chărgubilja (ichirgu-boil) Mostich, a contemporary of Simeon I and Peter I, from Preslav.
It has been hypothesized that Simeon's title was also recognized by a papal mission to Bulgaria in or shortly after 925, as a concession in exchange for a settlement in the Bulgarian-Croatian conflict or a possible attempt to return Bulgaria to union with Rome. Thus, in the later diplomatic correspondence conducted in 1199–1204 between the Bulgarian ruler Kaloyan and Pope Innocent III, Kaloyan — whose self-assumed Latin title was "imperator Bulgarorum et Blachorum" — claims that the imperial crowns of Simeon I, his son Peter I, and of Samuel were somehow derived from the Papacy. The Pope, however, only speaks of reges, kings of Bulgaria in his replies, and eventually grants only that lesser title to Kaloyan, who nevertheless proceeds to thank the Pope for the "imperial title" conferred upon him.
The title, later augmented with epithets and titles such as autocrat to reflect current Byzantine practice, was used by all of Simeon's successors until the complete conquest of Bulgaria by the Ottoman Empire in 1422. In Latin sources the Emperor of Bulgaria is sometimes designated "Emperor of Zagora" (with variant spellings). Various additional epithets and descriptions apart, the official style read "Emperor and autocrat of all Bulgarians and Greeks".
During the five-century period of Ottoman rule in Bulgaria, the sultan was frequently referred to as "tsar". This may be related to the fact that he had claimed the legacy of the Byzantine Empire or to the fact that the sultan was called "Basileus" in medieval Greek.
After Bulgaria's liberation from the Ottomans in 1878, its new monarchs were at first autonomous prince (knjaz). With the declaration of full independence, Ferdinand I of Bulgaria adopted the traditional title "tsar" in 1908 and it was used until the abolition of the monarchy in 1946. However, these titles were not generally perceived as equivalents of "Emperor" any longer. In the Bulgarian as in the Greek vernacular, the meaning of the title had shifted (although Paisius' Slavonic-Bulgarian History (1760–1762) had still distinguished between the two concepts). Accordingly, Ferdinand and his successors, Boris III and Simeon II, used the title of "king" in English. In the same fashion, the modern rulers of Greece used the traditional title of basileus in Greek and the title of "king" or "roi" in English and French.
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