Investigation and Analysis of the Role and Position of the City of Theodosiusopolis in the Relations Between Iran and Byzantium in the Sassanid Era

Armenia is among the Caucasian territories and has caused tensions between Iran and the Byzantine since its capture by the Parthians until the end of the Sassanid period. The continual fighting between two ancient powers over on this area caused that they devided Armenia between themselves in the year 387(A.D). After this division, the first Theodosius ordered that the border town of Theodosipolis be created in the western part of Armenia. The city was built on a hillside in the Karin area near the sources of the Euphrates River. With regard to these issues, this research seeks to address the geographical location of the city and then to continue its importance in the circle of Iran and Byzantium. To achieve this goal, the attempt has been made to use the method of historical research (Descriptive-analytical) and based on existing evidence and theories of researchers. The results of this study indicate that the Byzantines sought to build fortifications and tight fences in border cities such as Theodosipolis, in order to prevent the rapid advance of the Iranian army in the Byzantine territory, and even some times, the Sassanid lands could be Invaded from those cities. The Sassanians were able to attack the eastern lands of the Byzantine Empire by facilitating the capture of the city. The existence and strengthening of the fortresses in the city several times showed that the capture of the city constituted the domination and possession of the border regions, such as Asia Minor, and its loss as a major change in the balance of power between the two empires. 
Keywords: Armenia, Sassanid Empire, Byzantine Empire, Theodosius I, Theodosiusopoulos.
 
Introduction
Constantine I (337-306 AD), for the various reasons, most notably the position of geopolitics and Byzantine orientation for dealing with the Sassanian government, which had an aggressive policy on the western frontiers, took the capital from Rome to Byzantium (which Then he changed his name to Constantinople) in 330 AD. From the very beginning of the Third Century, the Sasanians had focused their foreign policy on expanding their territory in the West (Herodian, 1.26; Cassius, LXXX, 4 / 1-2; hence the center The confrontation between these two powers took place in the cities of the Caucasus, Asia Minor, Mesopotamia and Syria. The border towns that were attacked by both sides, each having their own significance and characteristics, have various construction and functions Political, military, economic, religious and geopolitical, which doubled the will of the two governments to capture them.  These factors, the claims of ownership on the lands of each other and the importance of the border towns between the two governments, together led to ongoing conflicts between the two powers, which expanded the scope of these conflicts in the capture and conversion of border towns in the west of the Sassanid Empire And the East of the Byzantine Empire.
Therefore, this paper, based on the importance of the subject and its nature, addresses the following questions, which are based on the basis of this research:
Research questions: 1. Who established Theodosipolis city and what was its geographic location? 2. What is the importance of the theodosipolis in the relations between Iran and Byzantium?
Research hypotheses: In response to these questions, the following hypotheses are raised:
Hypothesis 1:By attention to  given information and data provided by the resources available to us, the city of Theodosiusopolis was created by Theodosius I in the late fourth century and is geographically located in the western part of Armenia in a region called Karin is located near the sources of the Euphrates River.
Second hypothesis: on the one hand, it must be acknowledged that the most important function of the city of Theodosipolis, which was created by Byzantium, was politically motivated because, at the end of ancient times, the border towns like Theodosiusopolis laid the burden of defending the empire. On the other hand, by capturing the city, the Sassanians could have attacked the eastern lands of the Byzantine Empire more easily.
 
Identified Traces
The Theodosipolis geographic location
There is no controversy among the historians in relation to the geodesy of the city of Theodosipolis, and most of them express the city or region of Karin as the former city of Theodosiusopolis and believe that the city in the Karin region in a place previously inhabited by a village Which is called Kale Arkhe  in Greek (Eocene, 2003: 42).
 
Theodosipolis Town construction
Both in the sources and the researchers, there is no consensus on the founding of the city of Theodosiusopolis, some consider  the construction of  city during the time of Theodosius I (379-395 AD), while others consider it was established by the time of Theodosius II (408-450 AD) . Guards thought the reason for this disparity was  the inaccuracy of the relevant resources (Garsoevian, 2003: 64). But in this regard, according to the references to the sources and the indisputable proofs, it can be said this city was created after the split of Armenia in 387 AD between Iran and Byzantium during the time of Theodosius I.
 
The role of the city of Theodosipolis in the relations between Iran and Byzantium
The Byzantines, after the split of Armenia in the year 387, between themselves and Iran, aimed to advance their borders to build and strengthen the fortifications in the border towns in the western part of Armenia. Theodosipolis in the Romanian part of Armenia (western) is a sample of these cities, because it was located near the borders of Iran. The Sasanians also, were not passive to these fortifications and reacted more often, because strengthening the fortifications near their borders was a threat to their territory. Therefore, the Sassanid kings, when they were consolidating their internal affairs, attacked border towns, including the city of Theodosipolis.
 
Conclusion
From the very beginning, the Sasanians took an active foreign policy. The Byzantines, who did not show self-control over the Sassanian invasive policy, began to change the strategy. By establishing the fortifications in the border areas that were geopolitically important and by deploying sufficient forces there, they could partly prevent the rapid advance of the Iranian army in the Byzantine territory and could even invade these Sassanian territories from those cities.
The policy followed that the Byzantines founded the city of Theodosiusopolis (Karin and now Erzurum) near the borders of Iran. It was built during the theodosius I, in the part of the Armenian Roman (Western Armenia). The city’s most important function was the city’s political-military operation for the Byzantine Empire, since in the late ancient times, border towns such as Theodosipolis held the main burden of defending the empire.
The Sassanians, with the capture of the city, could attack the eastern lands of the Byzantine Empire more easily. As a result, there was a continuing struggle between them that followed the analysis of the power of the two empires.