Praetorian prefecture of Illyricum in the context of "Diocese of Thrace"

The Second Council of Constantinople is the fifth of the first seven ecumenical councils recognized by both the Eastern Orthodox Church and the Catholic Church. It is also recognized by the Old Catholics and others. Protestant opinions and recognition of it are varied. Some Protestants, such as Calvinists, recognize the first four councils, whereas Lutherans and most Anglo-Catholics accept all seven. Constantinople II was convoked by the Byzantine Emperor Justinian I under the presidency of Patriarch Eutychius of Constantinople. It was held from 5 May to 2 June 553. Participants were overwhelmingly Eastern bishops—only sixteen Western bishops were present, including nine from Illyricum and seven from Africa, but none from Italy—out of the 152 total.

The main work of the council was to confirm the condemnation issued by edict in 551 by the Emperor Justinian against the Three Chapters. These were the Christological writings and ultimately the person of Theodore of Mopsuestia (died 428), certain writings against Cyril of Alexandria's Twelve Anathemas accepted at the Council of Ephesus, written by Theodoret of Cyrrhus (died c. 466), and a letter written against Cyrillianism and the Ephesian Council by Ibas of Edessa (died 457).

Sirmium was a city in the Roman province of Pannonia, located on the Sava river, on the site of modern Sremska Mitrovica in the Vojvodina autonomous province of Serbia. First mentioned in the 4th century BC and originally inhabited by Illyrians and Celts, it was conquered by the Romans in the 1st century BC and subsequently became the capital of the Roman province of Pannonia Inferior. In 293 AD, Sirmium was proclaimed one of the four capitals of the Roman Empire. It was also the capital of the Praetorian prefecture of Illyricum and of Pannonia Secunda. The site is protected as an archaeological Site of Exceptional Importance. The modern region of Syrmia (Srem or Srijem) was named after the city.

Sirmium purportedly had 100,000 inhabitants and was one of the largest cities of its time. Colin McEvedy, whose estimates for ancient cities are much lower than the general consensus, put the population at only 7,000, based on the size of the archaeological site. The amount of grain imported between 1 AD and 400 AD was enough to feed 700,000 to 1 million people.

Flavius Julius Constans (c. 323 – 350), also called Constans I, was Roman emperor from 337 to 350. He held the imperial rank of caesar from 333, and was the youngest son of Constantine the Great.

After his father's death, he was made augustus alongside his brothers in September 337. Constans was given the administration of the praetorian prefectures of Italy, Illyricum, and Africa. He defeated the Sarmatians in a campaign shortly afterwards. Quarrels over the sharing of power led to a civil war with his eldest brother and co-emperor Constantine II, who invaded Italy in 340 and was killed in battle by Constans's forces near Aquileia. Constans gained from him the praetorian prefecture of Gaul. Thereafter there were tensions with his remaining brother and co-augustus Constantius II (r. 337–361), including over the exiled bishop Athanasius of Alexandria, who in turn eulogized Constans as "the most pious Augustus... of blessed and everlasting memory." In the following years he campaigned against the Franks, and in 343 he visited Roman Britain, the last legitimate emperor to do so until Manuel II in 1400, more than a thousand years later.

Justiniana Prima (Latin: Iustiniana Prima; Greek: Ιουστινιανή Πρώτη; Serbian: Јустинијана Прима, romanized: Justinijana Prima) was an Eastern Roman city that existed from 535 to 615 CE, near modern Lebane in the Leskovac region, Serbia. It is currently an archaeological site. Founded by Emperor Justinian I (527-565), it was the metropolitan seat of the newly founded Archbishopric of Justiniana Prima, which became the main church administrative body of the central Dardania with jurisdiction from Praevalitana to Dacia Ripensis. Justinian Prima was originally designed to become the capital of the prefecture of Illyricum, but for reasons likely related with its status near the Roman frontiers of the 6th century CE, Thessaloniki was preferred. It was abandoned less than 100 years after its foundation.

In 1979, the archaeological site of Justiniana Prima, identified with Caričin Grad, was added to the Archaeological Sites of Exceptional Importance-list under official protected status by the Republic of Serbia.