1. Internal Strife: The Parthian Empire faced continual internal conflicts and power struggles between various factions and claimants to the throne. This weakened the empire's stability and unity, making it vulnerable to external threats.
2. Economic Challenges: The Parthian economy was heavily reliant on trade, particularly with Rome. However, as Rome's demand for Parthian luxury goods declined and the Roman Empire expanded into the Middle East, the Parthian economy suffered, further straining the empire's resources.
3. Rise of the Sassanids: The Sassanids, a new dynasty from southern Iran, emerged as a formidable rival to the Parthians. They exploited the Parthians' internal weaknesses, gradually gaining control over territories and defeating the Parthian forces.
4. Military Defeats: In 224 CE, the Parthians suffered a decisive defeat at the hands of the Sassanid king Ardashir I. The Battle of Hormizdgan marked the end of the Parthian Empire, as the Sassanian dynasty established their rule over Iran.
5. Social and Cultural Factors: Some scholars suggest that the Parthian Empire also experienced social and cultural challenges, including tensions between the Parthian elite and the indigenous populations, as well as religious conflicts between Zoroastrians and other religious groups. These internal factors may have contributed to the empire's overall decline.
The Parthian Empire's fall marked the beginning of the Sassanian era in Iran and the transition from the Hellenistic to the Sassanian period in the Middle East. The Sassanids would rule for several centuries until the Arab conquests in the 7th century CE.