First Mongol Invasion (1241–1256)

The early 13th century was a turbulent time for the Second Bulgarian Empire, marked by internal strife and external threats. The reign of Tsar Boril (1207–1218) was characterized by significant challenges, including the strengthening of the Bogomil heresy, which prompted the first anti-Bogomil church council in 1211. Despite efforts to suppress heresy, Boril faced military setbacks, losing key allies such as Strez and suffering defeats against Latin and Lombard forces. Internal revolts, such as the one in Vidin, forced Boril to seek Hungarian assistance, resulting in territorial concessions around Belgrade and Braničevo to Hungary. The death of Emperor Henry of Thessalonica in 1216 further isolated Boril, leading to his eventual overthrow by Ivan Asen II.

Following the death of Ivan II Asen in 1241, Bulgaria entered a phase of dynastic transition from patrimonial succession to hereditary monarchy, exemplified by the succession of the young Kaliman I Asen. His minority necessitated a regency, during which Bulgaria sought to secure peace with its neighbors, including a two-year truce with the Latin Empire and treaties with the Empire of Nicaea. However, the Mongol threat loomed large, with "godless Tatars" invading Eastern Europe and reaching Bulgarian lands by 1243. The regency struggled to maintain stability, and Kaliman's early death in 1246 led to the accession of Michael II Asen, who was also a minor and ruled under regency.

The Mongol invasions intensified in the latter half of the 13th century, profoundly impacting Bulgaria's political landscape. By the 1270s and 1280s, the Bulgarian state had been territorially reduced, with the Golden Horde controlling lands north of the Danube and exerting significant influence over Bulgarian affairs. Tsar George I Terter, facing devastating Mongol raids in 1285, was compelled to accept vassalage under Khan Nogai, sending his son Theodore Svetoslav as a hostage to the Mongols and cementing alliances through marriage. This vassalage marked a shift from tributary relations to direct subordination, yet it also allowed Bulgaria to use Mongol military power as a diplomatic tool against regional rivals.

Simultaneously, Bulgaria's western and southern borders were under pressure from Hungary, Serbia, and Byzantium. Hungarian expansionism led to the loss of Braničevo and Vidin, with local Bulgarian nobles such as Darman and Kudelin establishing semi-independent rule in Braničevo before being defeated by Serbian forces allied with Hungary. The Serbian kingdom expanded into western Macedonia, capturing key Bulgarian-populated territories. Byzantium, after its restoration in 1261, sought to reclaim influence in Macedonia and Thrace, often allying with Mongol forces to counter Serbian advances.

The internal fragmentation of Bulgaria was exacerbated by the rise of powerful regional rulers like Alexius Slav in the Rhodope region, who acted as an autonomous despot and shifted allegiances between Bulgaria, Byzantium, and the Latin states. The Bulgarian state also saw the rise of the Terter dynasty, of Cuman origin, which ruled for over three decades and integrated Mongol diplomatic and military practices into Bulgarian governance.

The late 13th century witnessed attempts to form anti-Byzantine coalitions involving Bulgaria, Thessaly, and Western powers such as Charles I of Anjou. However, these efforts were undermined by the Sicilian Vespers uprising and Byzantine diplomatic maneuvers, including the deployment of Mongol cavalry to devastate northeastern Bulgarian lands. The complex interplay of alliances, vassalage, and military conflicts during this period set the stage for the gradual decline of Bulgarian power and the eventual Ottoman incursions in the 14th century.

Throughout these decades, Bulgaria's political and military fortunes were closely tied to its relations with the Mongols, Byzantines, Hungarians, and Serbs. The Mongol invasions forced Bulgaria into vassalage but also provided a means to counterbalance regional adversaries. The dynastic shifts from patrimonial to hereditary monarchy introduced new concepts of legitimacy, while internal divisions and external pressures eroded Bulgaria's territorial integrity and political autonomy, shaping the medieval Balkan geopolitical landscape.