444–450 (council – 8 August 449)

The condemnation of Eutyches and the "Robber Council" of Ephesus (449)

After the death of Cyril of Alexandria (444), his successor Dioscorus pursued a harsh policy against Antiochene Christology. He persecuted Cyril's friends and relatives. He found his opening in the case of the elderly Constantinopolitan archimandrite Eutyches. Eutyches taught that Christ had two natures before the union but only one after it, and denied that His flesh was consubstantial with ours.

A synod at Constantinople chaired by Patriarch Flavian (8–22 November 448) condemned and deposed Eutyches. With the backing of the powerful eunuch Chrysaphius, Eutyches secured a review of his case and appealed to Pope Leo, the emperor, and Dioscorus. Pope Leo sent Flavian his famous dogmatic letter (the "Tome"). It set out the doctrine of two natures united unconfusedly in the one Person of Christ.

At Theodosius II's insistence, a council convened at Ephesus on 8 August 449 under Dioscorus's chairmanship. The "Tome" was never read aloud. Eutyches was vindicated and restored, while Flavian and Eusebius of Dorylaeum were deposed (Flavian soon died from injuries in exile). Leading Antiochenes – Theodoret of Cyrus, Ibas of Edessa, and Patriarch Domnus of Antioch – were deposed as "Nestorians."

Theodosius II confirmed the council's decisions by edict and forbade any defense of the condemned. Pope Leo called the council a "den of robbers" (Latrocinium) and convened a council in Rome that condemned Dioscorus and Eutyches. But he could not secure a reversal while Theodosius lived.

The situation changed after the emperor's death (28 July 450). His successor Marcian, with his wife Pulcheria, rehabilitated the victims of 449 and agreed to convene a new ecumenical council.