After Theodosius II died on 28 July 450, the accession of the pro-western Marcian altered the political balance that had supported Dioscorus. Marcian and Pulcheria moved quickly to reverse Dioscorus’s ecclesiastical and theological achievements. A new general council was summoned to confirm the faith and met at Chalcedon on the Bosphorus opposite Constantinople in October 451.

More than 500 bishops participated in the council. Dioscorus was placed in a vulnerable position because he had defied Pope Leo and mistreated some of his clergy. He was deposed from his bishopric, although the council did not condemn him as a heretic. Eutyches was condemned again, and Alexandria’s political and ecclesiastical influence suffered a major setback.

The council adopted a Christological definition acceptable to the papal legates: Christ was to be acknowledged “in two natures inseparably united,” with each nature sharing the characteristics of the other. The assembled bishops celebrated the downfall of the Alexandrian “Pharaoh,” but the settlement did not end conflict among the major Christian centres of the Mediterranean.