Leo I the Thracian vs Theodosius the Great

Leo I was crowned Eastern Roman Emperor by Patriarch Anatolius of Constantinople. This was the first time a Byzantine emperor was crowned by a patriarch, establishing a precedent for the role of the Church in imperial coronations and strengthening the bond between church and state.

Leo I, in cooperation with the Western Emperor Anthemius, launched a massive naval expedition against the Vandal Kingdom. The campaign ended in a catastrophic defeat at the Battle of Cape Bon, costing the empire immense resources and weakening its military position.

Leo I ordered the assassination of the powerful Alan magister militum Aspar and his son, who had dominated the Eastern court for decades. This act ended the Germanic influence over the Eastern Roman army and solidified Leo's authority, but also led to a brief civil war.

Theodosius issued the Edict of Thessalonica, declaring Nicene Christianity the official state religion of the Roman Empire. All other Christian sects were deemed heretical, and pagan practices were increasingly suppressed.

Theodosius ordered a massacre in Thessalonica after a riot killed a Roman official. Thousands of citizens were killed in the circus. Bishop Ambrose of Milan excommunicated Theodosius until he performed public penance.

Theodosius defeated the usurper Eugenius and his general Arbogast at the Battle of the Frigidus. The victory reunited the Roman Empire under Theodosius's rule for the last time before its permanent division.

Upon his death, Theodosius divided the Roman Empire between his sons: Arcadius received the East and Honorius the West. This division became permanent, leading to the separate development of the Byzantine and Western Roman Empires.