Seleucia (; Ancient Greek: Σελεύκεια), also known as Seleucia-on-Tigris, Seleucia on the Tigris or Seleucia ad Tigrim, was a major Mesopotamian city, located on the west bank of the Tigris River within the present-day Baghdad Governorate in Iraq. It was founded around 305 BC by Seleucus I Nicator as the first capital of the Seleucid Empire, and remained an important center of trade and Hellenistic Seleucia (; Ancient Greek: Σελεύκεια), also known as Seleucia-on-Tigris, Seleucia on the Tigris or Seleucia ad Tigrim, was a major Mesopotamian city, located on the west bank of the Tigris River within the present-day Baghdad Governorate in Iraq. It was founded around 305 BC by Seleucus I Nicator as the first capital of the Seleucid Empire, and remained an important center of trade and Hellenistic culture after the imperial capital relocated to Antioch. The city continued to flourish under Parthian rule beginning in 141 BC; ancient texts claim that it reached a population of 600,000. Seleucia was destroyed in 165 AD by Roman general Avidius Cassius and gradually faded into obscurity in the subsequent centuries. The site was rediscovered in the 1920s by archaeologists. In 141 BC, the Parthians under Mithridates I conquered the city, and Seleucia became the western capital of the Parthian Empire. Tacitus described its walls, and mentioned that it was, even under Parthian rule, a fully Hellenistic city. Ancient texts claim that the city had 600,000 inhabitants, and was ruled by a senate of 300 people. It was clearly one of the largest cities in the Western world; only Rome, Alexandria, and possibly Antioch were more populous. In 55 BC, a battle fought near Seleucia was crucial in establishing dynastic succession of the Arsacid kings. In this battle between the reigning Mithridates III (supported by a Roman army of Aulus Gabinius, governor of Syria) and the previously deposed Orodes II, the reigning monarch was defeated, allowing Orodes to re-establish himself as king. In 41 Seleucia (; Ancient Greek: Σελεύκεια), also known as Seleucia-on-Tigris, Seleucia on the Tigris or Seleucia ad Tigrim, was a major Mesopotamian city, located on the west bank of the Tigris River within the present-day Baghdad Governorate in Iraq. It was founded around 305 BC by Seleucus I Nicator as the first capital of the Seleucid Empire, and remained an important center of trade and Hellenistic culture after the imperial capital relocated to Antioch. The city continued to flourish under Parthian rule beginning in 141 BC; ancient texts claim that it reached a population of 600,000. Seleucia was destroyed in 165 AD by Roman general Avidius Cassius and gradually faded into obscurity in the subsequent centuries. The site was rediscovered in the 1920s by archaeologists. In 141 BC, the Parthians under Mithridates I conquered the city, and Seleucia became the western capital of the Parthian Empire. Tacitus described its walls, and mentioned that it was, even under Parthian rule, a fully Hellenistic city. Ancient texts claim that the city had 600,000 inhabitants, and was ruled by a senate of 300 people. It was clearly one of the largest cities in the Western world; only Rome, Alexandria, and possibly Antioch were more populous. In 55 BC, a battle fought near Seleucia was crucial in establishing dynastic succession of the Arsacid kings. In this battle between the reigning Mithridates III (supported by a Roman army of Aulus Gabinius, governor of Syria) and the previously deposed Orodes II, the reigning monarch was defeated, allowing Orodes to re-establish himself as king. In 41 BC, Seleucia was the scene of a massacre of around 5,000 Babylonian Jewish refugees (Josephus, Ant. xviii. 9, § 9). In 117 AD, Seleucia was burned down by the Roman emperor Trajan during his conquest of Mesopotamia, but the following year it was ceded back to the Parthians by Trajan's successor, Hadrian, then rebuilt in the Parthian style. It was completely destroyed by the Roman general Avidius Cassius in 165. Following the edict of toleration by the Persian Sasanian King Yazdegerd I, which for the time being brought an end to the persecution of Christians, which had lasted for 70 years, the remaining Christians set about reorganizing and strengthening the church. The Council of Seleucia-Ctesiphon (The Synod of Mar Isaac) met in 410 AD under the presidency of Mar Isaac, the bishop of Seleucia-Ctesiphon. The most important decision of the Synod which had a very far reaching effect on the life of the church, was to declare the bishop of Seleucia-Ctesiphon as the primate of the Church of the East; and in recognition of this pre-eminence he was given the title 'Catholicos'. The Synod confirmed Mar Isaac as Catholicos and Archbishop of all the Orient. The Synod also declared its adherence to the decision of the Council of Nicaea and subscribed to the Nicene Creed. The Canons of the Synod leave no doubt as to the authority of the great Metropolitan, the Catholicos of Seleucia-Ctesiphon. Without his approval, no election of bishop would be valid. Towards the end of the reign of Yazdegerd I, the Christians in the Sasanian Empire were again persecuted in 420. Dadyeshu was elected Catholicos in 421 and himself suffered during the persecution and was imprisoned. When he was released he resigned and left Seleucia, but the church refused to accept the resignation and there followed the Synod of Dadyeshu, which met in 424 in Markabta of the Arabs under the presidency of Mar Dadyeshu. It proved to be one of the most significant of all Persian synods. The first synod of Isaac in 410 had decided that the Catholicos of Seleucia-Ctesiphon be supreme among the bishops of the East. The Synod of Dadyeshu decided that the Catholicos should be the sole head of the Church of the East and that no ecclesiastical authority should be acknowledged above him. For the first time, this synod referred to the Catholicos as Patriarch and that their Catholicos was answerable to God alone.