Chalcedon was called after a council at Ephesus in 449 had rehabilitated Eutyches and roughly handled his opponents, an assembly Leo of Rome called the robber synod. The new emperor Marcian, with Pulcheria, summoned a fresh council, and several hundred bishops gathered across the water from Constantinople, the largest such assembly the church had held.
The pivotal document was a letter. Leo had written to Flavian of Constantinople setting out a Christology of two natures in one person, each doing what is proper to it in communion with the other. Read at the council, it drew the acclamation that Peter had spoken through Leo, which is the single most quoted sentence in the history of papal claims and which the Eastern bishops who shouted it did not understand as the Latin church later would.
The definition that followed does not read like a creed. It rehearses the creeds of Nicaea and Constantinople, endorses Cyril's letters and Leo's tome, and then adds the minimum wording judged necessary: one and the same Christ, made known in two natures, without confusion, without change, without division, without separation, the difference of the natures not removed by the union. It is a fence rather than a portrait, drawn to keep two errors out, and the churches that received it have treated it as the boundary within which Christology must stay.