Abstract
The expanded territories of the ancient Iranian empires raise a question for the audience, at first glance, despite a display of authority and domination of the kings, that how the central governments extended their domination over these regions and defended it against foreign threats. In the wide Sasanian Empire, one policy of the emperors of the dynasty was to establish new cities and renew old ones to expand their dominance over the country. As a result of these efforts in which required the founding and reconstruction of intercity pathways for moving goods and troops, a vast network of cities and routes were gradually formed. In this network, cities had the role of nodes for controlling and providing the essential resources and supplies for the central government. Thus, it can assume that the network of cities, moreover expanding the dominance of the Sasanian government, also had affected their fall. This study aimed to find appropriate answers for this question, using library resources through a descriptive-analytical methodology that what role the communication network of Sasanian cities has played in the Arabs conquests. The result findings indicate that the vast communication network of the Sasanian cities has not only facilitated the Arabs troops’ movement and campaign, but they were also able to weaken the defensive power of cities through Blocking support routes. Moreover, The Arabs had been able to consolidate their occupancy and dominance by capturing the cities as network’s connection points.
Keywords: Sasanian, Arabs, Sasanian cities, Conquest, Communication network. 

Introduction
The concept of the realm can be defined by some individual domains played a source role for a central government interconnected via some access roads, and being protected from improvised border posts; however, each domain is surrounded by some enclosed areas not being purposed for settlement (Smith,2007:28-29). The ancient governments were modeled as some access networks to resources like cities, trade bases, or natural mines for their developed dominance based on the management of those charged operators in the controlled trades, taxes, infrastructures, law enforcement, and military proceedings (Smith,2005:835-836). In this way, the empire owns actually a communication network for transported goods and cities could be interlinked (Liverani,1988:86-92). Based on this model, the governors initially establish their authority over the most critical chosen resources, and subsequently, control the corridors and essential routes connecting the relevant domain to the imperial network system; so it is possible to construct some links as roads or canals from the new domain into other parts. Therefore, they can manage those realms with a high transformed economic and social data shown with their widespread control over the essential resources (Smith,2007:32-33). Facilitating the communication among various empiric sections, the Sasanian communication network plus their cities were widespread under the imposed state surveillance (Miri,2012:104). In the late period, monitoring the main network routes was assigned into the quadratic generalissimo (Spahbodān) of the Sasanian empire (Howard-Johnston,2012:125). It suggested a political and military significance of the network maintenance for the Sasanians, and according to Smith, after the intercity network of interaction finished, it was possible to collapse the states (Smith, 2005: 838). Given that the role of every Sasanian city was based upon certain matters and essentials, it is necessary to compare and analyze their effects in the fall of the Sasanian empire. The study findings are beneficial for those investigators of the historical urban background and the condition of traditional ancient Iran.
Therefore, this study aimed to evaluate the collapse of the Sasanian state and the Arab conquests based on the perspective of the network interconnection of Sasanian cities. The raised study question is that based on their status and place in the widespread network of Sasanian cities, what roles and effects had every Sasanian city placed along the routes of Arab conquests in this event? Library resources were used for data gathering in this descriptive-analytical study.

Network of Sasanian Cities and Roles in Arab Conquests
The Sasanians established their dominance policy based on the city developments and the necessity of their interconnection. Accordingly, surveillance of urban connection routes became essential for the state because of both military logistics and support and trade and revenue. That was highly important, especially in border regions and even some cities were established to support other cities in the frontlines (Liverani,1988:92). Besides the development of cities and communication routes, the states were considered the source access network with some defined regions and borders to be defended (Smith,2005:835). The potential danger of the Arabs made the Sasanians construct a defensive line of small military forts for surveilling important points in the communication network of the Mesopotamian plain (Howard-Johnston,2012:97-98). The Arabs used to violate the state frontiers. After the succession of Yazdgerd III, their motives increased for capture more areas after their progressive onrush into Iran’s territories (Tabari,2004:1587-1588). Intercity routes with a previous role for widespread Sasanian authority in Iranshahr cities were traversed by the Arabs and their troops to progress into the Persian central plateau (Tabari,2004:1959). Eastern Iran disconnected from the central cities after Rey’s capture (Frye,1977:12) as a critical route from the western cities to Khorasan (Nicolle,1996:12). Also, Zarang’s capture led to the disconnected Sistan-Khorasan due to the defeated highway of Zarang-Kirkuk-Herat (Ibn Huql,1987:158), and subsequently, Yazdgerd III was trapped in Khorasan (Farrokh,2013:92). Yazdgerd went to Marv city with special military strength and position in order to take back the monarchy, but Neishabour capture, the support military base, happened earlier by the Arabs (Bruner,2014:174). After the weakened defenses, the death of Yazdgerd III, and the peace of governor with the Arabs occurred in Marv city, the fall of the Sasanian dynasty was realized there.

Conclusion
The Sasanians expanded their authority through the developed urban centers and roads over their empire and then established strategic base points for land maintenance and protection. Due to the policy, cities like Neishabour in Khorasan or Mesopotamian Anbar were higher political and economic prominence over others being responsible for providing military and economic support to the surrounding cities. However, cities like Isfahan or Qazvin alongside Oboleh functioned as connection points and crossroads in the urban network, and sometimes, their removal meant simultaneous disconnection of several state routes from the control of the central government. Therefore, the Sasanians invested variously in building walls and ditches for cities’ protection, but political turmoil, civil wars in the late Sasanian period, and destructive effects of the long Byzantine wars overall weakened the defensive shield. Apparently, after invading Iran’s cities, the Arabs could use the joined Iranians to their armies, or the Iranian settled Arabs and identified well the routes and connection points of the cities, blocked the supply and support routes, and facilitated Iranshahr capture.