Founding of the Second Empire (1185–1197)

The Second Bulgarian Empire was founded in 1185 when the brothers Ivan Asen and Theodore-Peter initiated a rebellion against Byzantine domination, aiming to restore the Bulgarian state and unite all former Bulgarian lands. The uprising was inspired by the weakening of Byzantine power, notably after the Norman capture of Thessaloniki in 1185, which was interpreted as a divine sign of Byzantine decline. The brothers capitalized on the cult of Saint Demetrius of Thessaloniki, proclaiming that the saint had abandoned the city and taken refuge with the Bulgarians, thus providing spiritual legitimacy to their cause.

Initially, Theodore took the name Peter and was proclaimed Tsar, symbolizing the restoration of the Bulgarian kingdom. However, historical sources suggest a complex dual monarchy, with Ivan Asen also recognized as Tsar at some point during the early years. This dual rule reflected political disagreements between the brothers, with Peter favoring a more conciliatory approach towards Byzantium and Ivan Asen advocating for complete liberation and unification of all Bulgarian territories. The capital was established in Tarnovo, chosen for its symbolic and strategic significance, replacing the failed attempt to capture the old capital Preslav.

By 1186, the brothers had secured control over large parts of northern Bulgaria and parts of Thrace and Macedonia. The Byzantine Emperor Isaac II Angelos launched military campaigns against the uprising between 1186 and 1188 but was ultimately defeated, notably suffering a decisive loss in the Tryavna Pass in 1190. Ivan Asen emerged as the dominant ruler after Peter gradually withdrew to govern the region around Preslav, retaining nominal royal insignia but losing real power. Ivan Asen's reign until his assassination in 1196 was marked by consolidation and expansion, including diplomatic overtures to the Holy Roman Emperor and the Papacy to legitimize Bulgarian sovereignty.

Following Ivan Asen's death, the empire faced internal strife, including the assassination of both Ivan Asen and Peter by rivals such as Ivanko. The throne eventually passed to Kaloyan, who ruled from 1197 to 1207 and waged war against Byzantium, achieving a significant victory over the Latin Emperor Baldwin I near Adrianople in 1205. Kaloyan also sought recognition from Pope Innocent III, culminating in a union with the Roman Church in 1204, which was a strategic move to strengthen his position against Byzantium and the Latin Empire.

The empire continued to evolve under subsequent rulers, including Boril, Ivan II Asen, and others, facing challenges such as defeats by Latin emperors and invasions by Tatars. Ivan II Asen's reign (1218–1241) is noted for diplomatic activity, territorial expansion after the victory at Klokotnitsa in 1230, and the restoration of the Bulgarian Patriarchate in 1235, marking a peak in the empire's power and prestige. Despite territorial losses in later decades and internal conflicts, the Second Bulgarian Empire maintained its significance as a major Balkan power throughout the 13th century.

Throughout this period, the political program of the Asen dynasty was deeply influenced by the legacy of Tsar Simeon the Great, aiming to restore Bulgaria as a dominant imperial power in the region. This vision included the unification of Bulgarian lands, the assertion of imperial titles, and the establishment of an independent Bulgarian Orthodox Church. The empire's rulers skillfully balanced military campaigns, diplomacy, and religious policy to legitimize and consolidate their rule amidst the complex political landscape of medieval Southeastern Europe.