Chalcedon
The site of Chalcedon is located on a small peninsula on the north coast of the Sea of Marmara, near the mouth of the Bosphorus. A stream, called the Chalcis or Chalcedon in antiquity and now known as the Kurbağalıdere (Turkish: "stream with frogs"), flows into Fenerbahçe bay. There Greek colonists from Megara in Attica founded the settlement of Chalcedon in 685 BC, some seventeen years before Byzantium.
Marcian
To repudiate the Second Council of Ephesus, Marcian convened the Council of Chalcedon, the fourth council of the early church, in 451. Pulcheria may have influenced this decision, or even made the convention of a council a requirement during her negotiations with Aspar to marry Marcian. The council was to take place near Constantinople so that Marcian and Pulcheria could watch the proceedings closely. Initially, it was to be held at the city of Nicaea, which held enormous religious importance to the early church, as it was the site of their first council, the First Council of Nicaea in 325. However, Marcian successfully requested to transfer the spot to Chalcedon, because it was closer to Constantinople, and would thus allow him to respond quickly to any events along the Danube, which was being raided by the Huns under Attila. The council met in October 451, and was attended by about 370 bishops, most of them Eastern Roman, although four representatives were sent by Pope Leo I. This council reversed the decision of the Second Council of Ephesus that Jesus had one divine united nature, and instead agreed that Jesus had "a divine nature ("physis") and a human nature, united in one person ("hypostasis"), with neither division nor confusion".