If the doctrinal decisions of Ecumenical Councils are infallible, and since the Council of Chalcedon was an Ecumenical Council, how could the doctrinal decisions of the Council of Chalcedon be wrong? Can you explain why the first three Councils are considered Ecumenical while that of Chalcedon isn't?
The Ecumenical councils' decisions were taken by the Holy Spirit-inspired bishops present at the council to defend the Christian faith against the heresies of those times. We do not consider the council of Chalcedon Ecumenical; since there was no representation from the Church of Alexandria at that council. For, as soon as the members of the council had assembled, the legates of Rome demanded that Pope Dioscorus, the Patriarch of Alexandria, be banished on account of the order of the bishop of Rome whom they called, "the head of all churches".
The decisions of this council were influenced by the political powers of the time.