Robber Council of Ephesus
The Robber Council of Ephesus was convoked by Emperor Theodosius II on August 8, 449 for the purpose of adjudicating the findings of the council of November 448 chaired by Flavian of Constantinople that had deposed and excommunicated the Archimandrite Eutyches for refusing to admit the two natures of Christ. The council, under the chair of Dioscorus of Alexandria and support of Emperor Theodosius II, found in favor of Eutyches and deposed Flavian of Constantinople.
The fourth and fifth centuries witnessed an extensive period of controversies concerning the nature of Our Lord Jesus Christ, the Christological controversies. During this period a number of heresies were confronted including Arianism, Nestorianism, and Monophysitism. The disputes were argued amongst the hierarchs of the Churches of Alexandria, Antioch, and Constantinople in a number of councils (synods) with the Church of Rome represented by legates. The controversies were greatly influenced by emperor in Constantinople who generally convened the councils.
The first and third ecumenical councils established that Arianism and Nestorianism were heresies and excommunicated the principals, Arian and Nestorius. The excommunication and exile of Nestorius was greatly influence by the Empress Pulcheria, the older sister of Emperor Theodosius II, In 441, Theodosius, under the influence of the eunuch Chrysaphius, was convinced to dismiss his sister. Theodosius then began to support the Monophysite error that was promoted by the archimandrite Eutyches and Dioscorus, an error that Christ not only had one personality but also only one nature.
Domnus, Patriarch of Antioch, was the first to note this error by Eutyches. In November 448, Flavian, the Patriarch of Constantinople, convened a council in Constantinople during which a formal accusation of Eutyches’ error was made against him by Eusebius, Bishop of Dorylaeum (Phrygia). After hearing Eutyches, the council deposed and excommunicated Eutyches for refusing to admit two natures in Christ after the incarnation.
Upon appeal by Eutyches to the sympathetic emperor and other bishops, including Dioscorus, Theodosius convoked, with the agreement of Pope Leo of Rome, a council to determine whether in the 448 council Flavian had justly deposed and excommunicated Eutyches
The Second Council of Ephesus, as this council is also called, was convened by Emperor Theodosius II under the presidency of Dioscorus. Extant records of sessions read and recorded at the Council of Chalcedon in 451 and in a Syriac translation by a Monophysite monk provide a record of the council proceedings. The council was dominated by Theodosius, Dioscorus, and monophysitic supporters. Flavian and six bishops, who were present at the 448 council, were not allowed to sit as judges in the council. Dioscorus ignored the Roman legates and did not read the letter from Pope Leo, but he read a letter from Theodosius that directed the presence of a anti-Nestorian monk, Barsumas. Eutyches declared, after have been given the floor, that he held the faith of Nicene and Ephesus and that he was condemned by Flavian for a slip of the tongue, but he still stated the one nature of Christ. Attempts to present a case by the accuser of Eutyches, Bishop Eusebius of Dorylaeum, were refused, as well as by Flavian. Eutyches objected to attempts to read Leo’s letter, while a petition from Eutyches’ monastery, in his favor, was read. In the end Eutyches was declared orthodox and reinstated to his priestly office.
Flavian was deposed and exiled, soon to die of ill-treatment, and was succeeded by Anatolius, a deacon loyal to Dioscurus. Eusebius was also deposed. The council under Dioscurus then continued on to depose many bishops who had opposed him.
When reports of the actions of the Dioscurus led council and the violent actions of his partisans reached Pope Leo he condemned the council and called it the Latrocinium, a Robber Council and refused to recognize Anatolius as the lawful Bishop of Constantinople until he satisfactorily explained his belief. Theodosius, however, ignored the position of Leo. On July 28, 450, the situation changed radically as Theodosius was killed in a horse accident and his sister Pulcheria returned to imperial power and married the general Marcian, who became the new emperor. The empress and new emperor both opposed the teachings of Dioscurus and Eutyches. With the change in imperial leadership Anatolius and many other bishops now also condemned the teachings of Dioscurus and Eutyches. As a step to clarify the disputes Marician now called for a new council which was held in 451 in Chalcedon.
Worthy to note
It is beleived that many of the reasons that caused this great schism were political and also due to different interpretations of languages. The Coptic Orthodox Church never beleived in Monophysitism as it was claimed. It always beleived in Miaphysitism i.e. Christ is one from two natures. In other words, Christ is a full human and a full God, both natures are united without separation, confusion, or alteration. This can be noted in the confession part in the Coptic Divine Liturgy "... He made It one with His divinity without mingling, without confusion, and without alteration ..." ( 'It' refers to the Flesh taken from Saint Mary the Theotokos) which is what Cyril of Alexandria and Dioscorus of Alexandria said but was misunderstood to be in favour of Monophysitism or Eutychianism.
The dialogues between Oriental and Eastern Orthodox Churches started again in the 20th century after more than 15 centuries of seperation. During these dialogues, both parties found that they agree on many theological terms and definitions including this point that seperated them in the first place which came in favour to the opinions suggested that factors such as politics intervened in their seperation. More Details can found in "The story of the Copts" by Iris Habib El Masri. 
- Orthodox Unity - Statements from the Dialogue between the Eastern and Oriental Orthodox Churches
- Wikipedia: Iris Habib El Masri