His seven-year-old son Theodosius II succeeded Arcadius on the throne of the Western Roman Empire in 408. The new emperor often proved powerless and fragile. Yet, the presence of wise associates in the political arena ensured constructive decisions for the Empire. Influenced both by his sister Pulcheria and his wife Athinais-Eudocia - daughter of an Athenian philosopher - he took substantial measures in supporting letters. He reorganized the Supreme School of Constantinople - the famous University - where Greek and Latin grammar, rhetoric, philosophy was taught, and Latin law schools were established.

Another very important event was the publication of the so-called "Theodosian Codex" in 438. This was the first official compilation of all laws since the times of Constantine the Great. Court decisions in Greek were also allowed at the same time. Moreover, the defense of Constantinople was fortified during the reign of Theodosius II with new walls, reinforced by a deep trench and high towers.

Armenia became the main problem in foreign policy. The Byzantine inability to claim the whole of the territory from the Persians led them to a compromise. Yet, after relentless persecutions by the Persians against Christians, the Byzantine-Persian war broke out, ending with a peace treaty that proved temporary in retrospect. The Huns, spurred by Attila, their chief, made devastating raids in the Balkans during the same period. Despite diplomatic efforts and compromises on the part of Byzantium, the final crashing of Attila was only made possible in 451, when he was defeated by allied Western and Eastern forces.

The religious policy of Theodosius II gave rise to substantial theological disputes, which created upheavals among both clergy and people. The issue was the relation of the divine and human nature in Christ. One party supported the view of two separate natures in Christ, and another one the inextricable relationship between the two natures - divine and human. Nestorius, patriarch of Constantinople and a representative of the Antioch school, and Cyril, patriarch of Alexandria, were the champions of the two views. Even though supported by the emperor in the 3rd Ecumenical council in Ephesus in 431, Nestorius's view - which came to be known as Nestorianism - was condemned as heretic. Supported by archimandrite Eutychios in Constantinople, Monophysitism, a theory proclaiming the supremacy of the divine nature of Christ, emerged during the same period.