Presided over by Cyril, Patriarch of Alexandria, it condemned the notion that Mary was not the mother of God, the doctrine preached by the then bishop of Constantinople, Nestorius.  
In response to it a rival synod was set up under John of Antioch, which resulted in the excommunication of Memnon.  It had initially been called due to the dissatisfaction of Pope Celestine I, whose sway forced Emperor Theodosius II to convene it.  Presided over by Cyril, Patriarch of Alexandria, it condemned the notion that Mary was not the mother of God, the doctrine preached by the then bishop of Constantinople, Nestorius.  FTP, name this AD 431 Church Council in an Asia Minor city that inspired Paul to write an epistle preceding Galatians in the New Testament.
In response to it a rival synod was set up under John of Antioch, which resulted in the excommunication of Memnon.  
Council of Ephesus