Otto I the Great vs Theodosius I

Otto married Adelaide, the widowed queen of Italy, after intervening in Italian politics. This marriage gave him control over the Kingdom of Italy and strengthened his claim to imperial authority.

Otto led a German army to defeat the Magyar (Hungarian) forces at the Lechfeld near Augsburg. This victory ended Magyar raids into Western Europe and secured Otto's reputation as a defender of Christendom.

Pope John XII crowned Otto I as Holy Roman Emperor in Rome, reviving the imperial title in the West. This event established the Holy Roman Empire as a major political entity and linked German kingship with papal authority.

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

Theodosius I ordered a massacre of thousands of citizens in Thessalonica in retaliation for the murder of a Roman general. The massacre led to his excommunication by Bishop Ambrose of Milan, and he later performed public penance.

Theodosius I issued a series of laws banning pagan worship, closing temples, and prohibiting sacrifices. The Olympic Games were abolished, and the Serapeum in Alexandria was destroyed. These actions accelerated the decline of paganism in the empire.

Theodosius I defeated the usurper Eugenius and his general Arbogast at the Battle of the Frigidus (modern River Vipava). The victory reunited the Roman Empire under a single ruler for the last time before its permanent division.

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