The once-mighty Seleucid kingdom had in the beginning of the third century BCE begun to lose control over large territories.

Parthia, Bactria, and Sogdiana have by the middle of this century gained their independence.

The Middle East: 249–238 BCE

Parthian Emergence and Hellenistic Decline

Rise of Arsaces and the Parthian State

In 247 BCE, following the death of Antiochus II, Seleucid control weakens as their governor (satrap) of Parthia, Andragoras, declares independence amidst the turmoil caused by the seizure of the Seleucid capital Antioch by Ptolemy III. Andragoras struggles to defend his territory without Seleucid military backing.

Around 238 BCE, the situation deteriorates further when Arsaces, a leader of the nomadic Parni tribes of Scythian or Bactrian origin, launches a decisive invasion into Parthia, aided by his brother Tiridates. Quickly capturing Astabene (Astawa) and its capital, Kabuchan (modern Kuchan), the Parni decisively end Andragoras' rule, killing him in the process. The Parni tribes subsequently adopt the name Parthians, derived from the conquered province, marking the birth of a new and influential Persian dynasty.

Advances in Hellenistic Astronomy

Amidst these geopolitical shifts, significant advancements occur in Greek astronomy. Aristarchus of Samos, a prominent student of Strato of Lampsacus, advocates for a revolutionary heliocentric model, asserting that the Sun, not the Earth, is the center of the universe. Aristarchus also conducts pioneering work in determining celestial distances. In his surviving treatise, On the Sizes and Distances of the Sun and Moon, he uses geometric reasoning to calculate that the Sun is approximately twenty times further away and twenty times larger than the Moon. Although his estimates are imprecise due to technological limitations, Aristarchus's methods remain conceptually sound and significantly influence future astronomical thought.

Andragoras, the Seleucid governor (satrap) of Parthia (roughly, western Khorasan) had proclaimed independence from the Seleucids in 247 BCE, when—following the death of Antiochus II—Ptolemy III had seized control of the Seleucid capital at Antioch.

Meanwhile, one Arsaces, of Scythian or Bactrian origin, has been elected leader of the Parni tribes.

Following the secession of Parthia from the Seleucid Empire and the resultant loss of Seleucid military support, Andragoras has difficulty in maintaining his borders, and about 238 BCE—under the command of Arsaces and his brother Tiridates—the Parni invade Parthia and seize control of Astabene (Astawa), the northern region of that territory, the administrative capital of which is Kabuchan (Kuchan in the vulgate).

The Parni shortly seize the rest of Parthia from Andragoras, killing him in the process, and soon become known as the Parthians, taking their name from the Seleucid province that they have conquered.

The Seleucid Empire, founded in 323 BCE by Seleucus I Nicator, a general of Alexander the Great, stretches from Syria to the Indus River and comprises most of Alexander's realm.

The Seleucid state was the most powerful of the Diadochi kingdoms that sprang up after Alexander's death.

Quickly however, the Seleucids had run into trouble trying to maintain such an extended realm, facing constant warfare against the other Hellenistic states in the west and with unrest among their Iranian peoples in the east.

Diodotus and Andragoras, the Seleucid satraps of Bactria and Parthia respectively, had around 245 BCE declared their remote provinces independent states, taking advantage of the Seleucids' preoccupation with the wars against a Celtic invasion of Asia Minor in the west.

The Parni, one of three nomadic or seminomadic Iranian tribes in the confederacy of the Dahae living east of the Caspian Sea, had apparently moved southward into the region of Parthia after the death of Alexander the Great in 323 BCE and perhaps eastward into Bactria.

They seem to have adopted the speech of the native Parthians, a language loosely related to Scythian and Median, and been absorbed into the settled population.

According to tradition (somewhat disputed), Arsaces, a governor under Diodotus who is of Scythian or Bactrian origin (and may even have been a Bactrian prince), revolts and flees westward to establish his own rule.

The Parni soon become known as the Parthians, taking their name from the Seleucid province that they have conquered.

They now began to try and conquer as much of the eastern Seleucid empire as possible, joined in this by the now independent province of Bactria.

The Seleucid king Antiochus II Theos was at the time too busy fighting a war against Ptolemaic Egypt; thus, the Seleucids had lost most of their territory east of Persia and Media.

With the Parni, Arsaces in about 238 BCE seizes Astauene (or Astabene), i.e., northern Parthia.

Andragoras is killed during his attempts to recover it, which leaves the Parni in control over the rest of Parthia as well.

A recovery expedition by the Seleucids under Seleucus II is unsuccessful, and Arsaces and the Parni will succeed in holding Parthia proper during Arsaces' lifetime. (Some authorities believe that a brother, Tiridates I, succeeded Arsaces about 228 and ruled until 211; other authorities consider Arsaces I and Tiridates I to be the same person.)

The Middle East: 237–226 BCE

Consolidation of the Parthian Kingdom

Seleucid-Parthian Conflict and Diplomacy

The emergence of two new kingdoms in Iran, Arsaces I’s Parthian Kingdom and the Greco-Bactrian kingdom of Diodotus, occurs nearly simultaneously, though notable differences distinguish them. The Parthian kingdom is closely tied to the tribal groups inhabiting northern Iran, combining settled populations with nomadic neighbors. Unlike in Bactria, the Greek cultural influence in Parthia remains comparatively modest, playing a less pivotal role in governance and society.

Around 232–231 BCE, Seleucid ruler Seleucus II launches a campaign to reassert control over the rebellious Parthians. Facing Seleucus’ superior forces, Arsaces, maintaining close alliances with northern nomads, strategically withdraws to the territory of the Apasiacae, or “Scythians of the Waters.” Seleucus attempts to pursue Arsaces across the Jaxartes River (present-day Syr Darya), but encounters stiff resistance from nomadic tribes, resulting in significant losses.

Arsaces’ State-Building and Legacy

Receiving alarming news of threats from the west, Seleucus chooses to retreat to Syria, opting instead for a diplomatic resolution. A peace agreement is reached, under which Arsaces recognizes Seleucid suzerainty, while effectively securing his kingdom's autonomy.

From this point forward, Arsaces significantly alters his approach, transitioning from nomadic leadership to that of a structured state ruler, emulating the organizational style of the Seleucids. He formally crowns himself king and establishes or fortifies cities such as Asaak and the impregnable fortress Dara, as well as founding the city of Nisa, which later serves as his burial place. These new settlements, frequently named in honor of the king or his dynasty, are designed to solidify his power and centralize governance.

While Arsaces maintains respect for the autonomy of Greek and Macedonian communities within his domain—likely a calculated effort to retain their support—he utilizes carefully managed propaganda to ensure their ongoing allegiance. Arsaces sets up his capital at Hecatompylos, strategically located southeast of the Caspian Sea along the Silk Road in western Khurasan. Although the exact site remains uncertain, it is believed to lie between modern-day Iranian cities Damghan and Shahrud.

The Arsacid dynasty begins the ambitious task of recovering territories that once belonged to the Persian Empire, organizing their realm into semi-autonomous states governed by a feudal nobility. This nobility, while acknowledging Arsacid supremacy, enjoys considerable regional autonomy, shaping the distinctive decentralized political structure of the Parthian Empire.