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Development of Moravia (c. 800–846)

Moravia, the first Western Slavic polity, arose through the unification of the Slavic tribes settled north of the Danube. However, its formation is scarcely described by contemporaneous sources. The archaeologist Barford writes that the first report of the emerging Moravian state was recorded in 811. In the autumn of this year, according to the Royal Frankish Annals, Avar rulers and the duces or "leaders of the Slavs who live along the Danube" visited the court of Emperor Louis the Pious (r. 814–840) in Aachen. The earliest certain reference to Moravians or Maravani is dated to 822 when the emperor "received embassies and presents from all the East Slavs, that is, Obodrites, Sorbs, Wilzi, Bohemians, Moravians and Praedenecenti, and from the Avars living in Pannonia " at an assembly held at Frankfurt.

The late-9th-century Conversio Bagoariorum et Carantanorum ("The Conversion of the Bavarians and the Carantanians") makes the first reference to a Moravian ruler. Carantanians (ancestors of present-day Slovenians) were the first Slavic people to accept Christianity from the West. They were mostly Christianized by Irish missionaries sent by the Archdiocese of Salzburg, among them Modestus, known as the "Apostle of Carantanians". This process was later described in the Conversio Bagoariorum et Carantanorum, which states that Mojmír, "duke of the Moravians", expelled "one Pribina " across the Danube. Pribina fled to Ratpot who administered the March of Pannonia from around 833. Whether Pribina had up to that time been an independent ruler or one of Mojmir's officials is a matter of scholarly discussion. For instance, Urbańczyk writes that Mojmir and Pribina were two of the many Moravian princes in the early 9th century, while according to Havlík, Třeštík and Vlasto, Pribina was Mojmír's lieutenant in Nitra. Historians who identify Pribina as the ruler of an autonomous state, the Principality of Nitra —for instance, Bartl, Kirschbaum and Urbańczyk —add that "Great Moravia" emerged through the enforced integration of his principality into Moravia under Mojmír.

The 9th-century Catalogue of Fortresses and Regions to the North of the Danube —which lists the peoples along the borders of East Francia in a north-to-south order—mentions that the Moravians or Marharii had 11 fortresses or civitates. The document locates the Marhari between the Bohemians and the Bulgars, and also makes mention of the Merehani and their 30 fortresses. According to Havlík, who writes that Conversion is a consolidated version of notes made by several authors in different years, the Moravians are twice mentioned in the text: first as Marhari, and next as Merehani. He says, that the reference to the Marhari and their 11 fortresses was made between 817 and 843, and the note of the Merehani shows the actual state under Svatopluk I. In contrast with Havlík, Steinhübel together with Třeštík and Vlasto identify the Merehani with the inhabitants of the Principality of Nitra. A third view is presented by Püspöki-Nagy and Senga, who write that the reference to the Merehanii —who obviously inhabited the southern regions of the Great Hungarian Plains to the north of the Danube, but south of the territories dominated by the Bulgars—and their 30 fortresses shows the existence of another Moravia in Central Europe.

Among the Bohemians are 15 fortresses. The have 11 fortresses. The region of the Bulgars is immense. That numerous people has five fortresses, since their great multitude does not require fortresses. The people called have 30 fortresses.

According to a 13th-century source, the History of the Bishops of Passau and the Dukes of Bavaria, Bishop Reginhar of Passau (r. 818–838) baptized "all of the Moravians" in 831. There is no other information on the circumstances of this mass conversion. Vlasto writes that Mojmír had by that time been converted to Christianity; according to Petr Sommer and other historians, he was also baptized on this occasion. All the same, the Life of Methodius narrates that Christian missionaries had by the 860s arrived in Moravia "from among the Italians, Greeks and Germans " who taught them " in various ways ". The Life of Constantine adds that missionaries from East Francia did not forbid "the offering of sacrifices according to the ancient customs", which shows that pagan rites were continued for decades even after 831.

According to the Annals of Fulda, around August 15, 846, Louis the German, King of East Francia (r. 843–876) launched a campaign "against the Moravian Slavs, who were planning to defect". The exact circumstances of his expedition are unclear. For instance, Vlasto writes that the Frankish monarch took advantage of the internal strife which followed Mojmír's death, while according to Kirschbaum, Mojmír was captured and dethroned during the campaign. However, it is without doubt that Louis the German appointed Mojmír's nephew, Rastislav, as the new duke of Moravia during this campaign.