18 But St. Leo's principal title to fame is the Council of Chalcedon, held in 451. In spite of pressure from the Emperor Marcian, the Pope refused to allow it to be summoned except on condition that his own legates should preside over it. (16) It proved, Venerable Brethren, to be one of the greatest events in the history of the Church, renowned alike for its solemn definition of the doctrine of the two natures in God's Incarnate Word, and its recognition of the magisterial primacy of the Roman Pontiff. We need not, however, enter into any more detailed discussion of it here, for Our predecessor Pius XII has already dealt with it in an important Encyclical addressed to the entire Catholic world on the fifteenth centenary of its convocation. (17)
Source: Aeterna Dei Sapientia (Vatican.va)