After receiving the Mesopotamian regions of Babylonia and Assyria in 321 BC, Seleucus I began expanding his dominions to include the Near Eastern territories that encompass modern-day Iraq, Iran, Afghanistan, Syria, and Lebanon, all of which had been under Macedonian control after the fall of the former Achaemenid Empire.

To the east, conflict with the Indian ruler Chandragupta of the Maurya Empire in 305 BC led to the cession of vast territory west of the Indus and a political alliance.

The Seleucids were forced to pay costly war reparations and had to relinquish territorial claims west of the Taurus Mountains in southern Anatolia, marking the gradual decline of their empire.

Mithridates I of Parthia conquered much of the remaining eastern lands of the Seleucid Empire in the mid-second century BC, including Assyria and what had been Babylonia, while the independent Greco-Bactrian Kingdom continued to flourish in the northeast.

Contemporary sources, such as a loyalist decree honoring Antiochus I from Ilium, in Greek language define the Seleucid state both as an empire (ἀρχή, archḗ) and as a kingdom (βασιλεία, basileía).

According to Appian: He [Seleucus] crossed the Indus and waged war with Sandrocottus [Maurya], king of the Indians, who dwelt on the banks of that stream, until they came to an understanding with each other and contracted a marriage relationship.

In addition to this treaty, Seleucus dispatched an ambassador, Megasthenes, to Chandragupta, and later Deimakos to his son Bindusara, at the Mauryan court at Pataliputra (modern Patna in Bihar state).

Later Ptolemy II Philadelphus, the ruler of Ptolemaic Egypt and contemporary of Ashoka the Great, is also recorded by Pliny the Elder as having sent an ambassador named Dionysius to the Mauryan court.

Towards the end of Antiochus II's reign, various provinces simultaneously asserted their independence, such as Bactria and Sogdiana under Diodotus, Cappadocia under Ariarathes III, and Parthia under Andragoras.

[citation needed] The Seleucid economy started to show the first signs of weakness, as Galatians gained independence and Pergamum took control of coastal cities in Anatolia.

Although initially unsuccessful in the Fourth Syrian War against Egypt, which led to a defeat at the Battle of Raphia (217 BC), Antiochus would prove himself to be the greatest of the Seleucid rulers after Seleucus I himself.

He spent the next ten years on his anabasis (journey) through the eastern parts of his domain and restoring rebellious vassals like Parthia and Greco-Bactria to at least nominal obedience.

Actual translation of Polybius 11.34 (No other source except Polybius makes any reference to Sophagasenus): He [Antiochus] crossed the Caucasus Indicus (Paropamisus) (Hindu Kush) and descended into India; renewed his friendship with Sophagasenus the king of the Indians; received more elephants, until he had a hundred and fifty altogether; and having once more provisioned his troops, set out again personally with his army: leaving Androsthenes of Cyzicus the duty of taking home the treasure which this king had agreed to hand over to him.

Antiochus and Philip V of Macedon then made a pact to divide the Ptolemaic possessions outside of Egypt, and in the Fifth Syrian War, the Seleucids ousted Ptolemy V from control of Coele-Syria.

[36] On his return journey, according to Josephus, he made an expedition to Judea, took Jerusalem by force, slew a great many who had favored Ptolemy, sent his soldiers to plunder them without mercy.

Demetrius I attempted to restore Seleucid power in Judea particularly, but was overthrown in 150 BC by Alexander Balas—an impostor who (with Egyptian backing) claimed to be the son of Epiphanes.

Once-vassal dynasties in Armenia, Cappadocia, and Pontus were threatening Syria and northern Mesopotamia; the nomadic Parthians, brilliantly led by Mithridates I of Parthia, had overrun upland Media (home of the famed Nisean horse herd); and Roman intervention was an ever-present threat.

Sidetes managed to bring the Maccabees to heel and frighten the Anatolian dynasts into a temporary submission; then, in 133, he turned east with the full might of the Royal Army (supported by a body of Jews under the Hasmonean prince, John Hyrcanus) to drive back the Parthians.

Despite being a tiny minority of the overall population, these Greeks were the backbone of the empire: loyal and committed to a cause that gave them vast territory to rule, they overwhelmingly served in the military and government.

Synthesizing Hellenic and indigenous cultural, religious, and philosophical ideas – an ethnic unity framework established by Alexander – met with varying degrees of success.

Antiochus IV's violent retaking of the city and the banning of traditional Judean practices led to the eventual loss of control of Judea by the Seleucid government, paving the way for the rise of an independent Hasmonean kingdom.

[55] Following losses of territory in Asia Minor during the Roman-Seleucid War, King Antiochus IV sponsored a new wave of immigration and settlements to replace them and maintain enough Greeks to staff the phalanxes seen at the military parade at Daphne in 166–165 BC.

[57][58][59][60] While the motive is simple enough, the Seleucid Empire boasts of a sophisticated political economy that extracts wealth from local temples, cities (or poleis), and royal estates; much of which was inherited from their Achaemenid predecessors.

[60] Rather, the introduction and widespread implementation of currency is attributed to Darius I's tax reforms centuries prior;[60] hence, the Seleucids see a continuation rather than shift in this practice, i.e. the payment of taxation in silver or, if necessary, in kind.

[61] The minute difference in weight between a shekel and didrachm (weighing 8.6 g silver) could not be expressed in this barter system, and a Greek tetradrachm would be "a far too heavy denomination...in daily trade.

[60] Traditional "primitivist" narratives of the ancient economy argue that it was "marketless"; however, the Babylonian astronomical diaries show a high degree of market integration of barley and date prices—to name a few—in Seleucid Babylonia.

In an extreme example, Spek believes tribal Arab raiding into Babylonia caused barley prices to skyrocket to 1493 grams of silver per ton from 5–8 May, 124 BC.

"[57] As a hegemonic empire, the state's primary focus was maintaining its sizable army via wealth extraction from three major sources:[59] tribute from autonomous poleis and temples, and proportional land-tax from royal land.

[58] Although the Seleucid kings were aware and appreciated the sacrosanctity of religious treasures, their concentration in these places "proved irresistible" in the face of "short-term fiscal constraints.

[58] According to historian Michael J. Taylor:[58]It is difficult to believe that these monarchs who knew enough to bow before Nabu, bake bricks for Esagil, and enforce kosher regulations in Jerusalem, would be blithely aware of the political hazards of removing Temple treasures.