Parthians were from the Aryan tribes were living in the province of Parth. During the Achaemenid era, the province of Parth has included the current Khorasan province and part of Turkmenistan. Despite of the 500 years of rule of this empire in Iran, there is less available information about them in comparison to before and after empires.
In 247 BC, when the Seleucid Empire was involved in the war in the West and the war with Egyptians in Egypt, the founder of the Parthian Empire, Arsaces I took the opportunity and set up a small empire for himself and his tribe. After the ruling of many kings who sat on the throne, a great monarchy named Mehrdad I (from 171 to 138 BC) sat on the throne. He was one of the most powerful Parthian kings who made Parthians one of the great empires in the world by conquering important cities like Bactria, Pars, and Babylon and defeating Demetrius III the second king of the Seleucids who ruled in Syria. After Mehrdad I, many kings did not do any important actions. Until the turning point came to the most powerful monarch of the Parthian king, Mehrdad II. He defeated the Scythians who defied and restored the eastern territories of the empire and set political relations with the Roman Empire.
He was the first monarch of the Parthian Empire who like the Achaemenid kings, tagged himself as the king of the kings, which was due to his determination to rebuild Iranian culture after the Seleucids. Goudarz, Orodes I, Santaruk, Farhad IV, and Mehrdad III were the next kings of Parthian Empire. After that, Orodes III became the king of Iran, one of the most important events of his period was the important War against the Romans, which led to the big defeat of the Roman armies, which was written by Plutarch, Parthian armies behead Marcus Licinius Crassus and bring his head to Orodes while he was watching a theatre with the King of Armenia.
During the ruling of the next kings of Parthian, there were always conflicts between Parthians and Roman Empire over Armenia. The conflicts with the Romans over the years and the over and over wars eventually led to the death of Artabanus V, the last king of the Parthians who died in Ctesiphon. This great dynasty became extinct after about 500 years of rule, and in 224 AD it was replaced by the Sasanians.