Maximinus, a member of the Ancient / Classical History forum, asked how the Persian Empire (modern Iran) managed to survive for as long as it did. Another forum member, KL47, answers this question:
The original Persian (or Achaemenid) empire, as established by Cyrus the Great in the 6th century B.C., only lasted approximately 200 years until the death of Darius III in 330 B.C., following his defeat by Alexander the Great. The core territories of the empire were then ruled by Macedonian dynasties, primarily the Seleucids, until the late 2nd century BC. During the early 2nd century B.C., however, the Parthians (who were not Persians but rather descended from a branch of the Scythians) set up a new kingdom in eastern Iran, originally in a breakaway province of the Seleucid empire. Over the next half-century, they gradually took over much of the rest of what had once been Persian-controlled territory, adding Media, Persia, and Babylonia to their holdings. Roman writers of the early imperial period sometimes refer to this or that emperor going to war with "Persia", but this is really a poetic or archaic way of referring to the Parthian kingdom.
The Parthians (also referred to as the Arsacid dynasty) remained in control until the early 3rd century A.D., but by that time their state was seriously weakened by in-fighting and they were overthrown by the native Persian Sassanid dynasty, who were militant Zoroastrians. According to Herodian, the Sassanids laid claim to all the territory once ruled by the Achaemenids (much of which was now in Roman hands) and, at least for propaganda purposes, decided to pretend that the 550+ years since the death of Darius III had never happened! They continued to whittle away at Roman territory for the next 400 years, eventually coming to control most of the provinces once ruled by Cyrus et al. This all fell apart, however, when the Roman emperor Heraclius launched a successful counter-invasion in A.D. 623-628, which threw the Persian state into total chaos from which it never recovered. Shortly afterwards, the Muslim hordes invaded and Persia lost its independence until the 16th century when the Safavid dynasty came to power.
Facade of Continuity
The Shahs of Iran maintained the pretense of an unbroken continuity from the days of Cyrus, and the last one held a huge pageant in 1971 to celebrate the 2500th anniversary of the Persian empire, but he wasn't fooling anyone familiar with the history of the region.
Persia Longevity Question
"Has anybody noticed that the Persian Empire seems to have eclipsed all others or is that just my imagination? I refer to the fact that Persia was a great power in 400 B.C. and controlled much of the Ionian coast. But we also hear of Persia much later at the time of Hadrian and, by all accounts, Rome avoided prolonged conflict with this rival power." Maximinus