Serbia

The early 14th century saw a significant diplomatic development between Bulgaria and Serbia, marked by the marriage of Ivan Alexander's sister Elena to King Stefan Dušan on April 19, 1332. This union ended potential hostilities and established a lasting peace that endured until the Ottoman conquest. While Bulgaria secured its western borders and gained a degree of security, it came at the cost of relinquishing large Bulgarian-populated territories in Macedonia to Serbia, whose national identity remained resilient for centuries. This peace effectively divided spheres of influence, leaving Thrace to Bulgaria and the rest of the peninsula to Serbia, a division that favored Serbian expansion.

Serbia's expansion into Macedonia was facilitated by the internal strife within the Byzantine Empire and Bulgaria's preoccupation with Thrace. Stefan Dušan capitalized on Byzantine civil wars and Bulgarian distractions to annex vast territories, including almost all of Macedonia by the mid-14th century. His coronation as "Emperor of Serbs and Greeks" in 1346, blessed by the Bulgarian Patriarch Simeon, symbolized Serbia's newfound power and the emergence of a third Balkan empire alongside Bulgaria and Byzantium. Bulgaria's passive stance and even tacit support for Serbian advances highlighted the declining influence and strategic miscalculations of Ivan Alexander.

Internally, Bulgaria faced fragmentation and decentralization, with appanage principalities like Vidin under Ivan Sratsimir and Dobruja under Dobrotitsa gaining autonomy and eventually asserting independence. The political disunity weakened Bulgaria's ability to respond effectively to external threats. Meanwhile, the rise of the Ottoman Turks introduced a new and formidable adversary. Initially allied with Byzantine factions, the Turks soon turned hostile, launching devastating raids and establishing footholds in Thrace, further destabilizing the region.

Bulgaria's foreign policy during Ivan Alexander's reign was marked by missed opportunities and diplomatic failures. Despite efforts to forge alliances, including marriage ties with Byzantium and cooperation with Venice and Genoa, Bulgaria could not form a united front against the Ottoman threat. Ivan Alexander's reluctance to join joint anti-Turkish coalitions, influenced by his son-in-law Stefan Dušan, proved detrimental. The loss of strategic territories such as Macedonia and parts of Thrace, coupled with the inability to prevent Hungarian incursions into Vidin, underscored the empire's weakening position.

The mid-14th century also witnessed the temporary Hungarian conquest of Vidin (1365–1369), during which King Louis I imposed Catholicism and established administration, further complicating Bulgaria's internal cohesion. Ivan Alexander managed to orchestrate a coalition that expelled the Hungarians, but the episode revealed the empire's vulnerability. The subsequent years saw increasing Ottoman advances, the fall of key cities, and the fragmentation of Bulgarian authority.

Following Ivan Alexander's death in 1371, Bulgaria was divided among his sons, with Ivan Shishman ruling Tarnovo and Ivan Sratsimir governing Vidin as a quasi-independent ruler. This division weakened Bulgaria's capacity to resist Ottoman expansion. The final decades of the 14th century were dominated by Ottoman conquests, internal strife, and failed uprisings, culminating in the fall of Tarnovo in 1393 and Vidin in 1396. Attempts at liberation, such as the uprising led by Ivan Shishman's son Fruzhin and Konstantin, persisted into the early 15th century but ultimately failed.

Throughout this period, the interplay of dynastic marriages, shifting alliances, and external pressures shaped the fate of the Second Bulgarian Empire. Serbia's rise under Stefan Dušan, Bulgaria's territorial concessions, and the looming Ottoman threat redefined Balkan geopolitics, leading to the eventual demise of medieval Bulgarian statehood and the transformation of the region under Ottoman rule.