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). Similarly, Seleucid rulers were described as kings in Babylonia.

The rulers did not describe themselves as being of any particular territory or people, but starting from the 2nd century BC, ancient writers referred to them as the Syrian kings, the kings of Syria or of the Syrians, the kings descended from Seleucus Nicator, the kings of Asia, and other designations.

The domain of the Seleucids stretched from the Aegean Sea to what is now Afghanistan and Pakistan, therefore including a diverse array of cultures and ethnic groups. Greeks, Assyrians, Armenians, Georgians, Persians, Medes, Mesopotamians, Jews, and more all lived within its bounds. The immense size of the empire gave the Seleucid rulers a difficult balancing act to maintain order, resulting in a mixture of concessions to local cultures to maintain their own practices while also firmly controlling and unifying local elites under the Seleucid banner.

The government established Greek cities and settlements throughout the empire via a program of colonization that encouraged immigration from Greece; both city settlements as well as rural ones were created that were inhabited by ethnic Greeks. These Greeks were given good land and privileges, and in exchange were expected to serve in military service for the state. 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. Unlike Ptolemaic Egypt, Greeks in the Seleucid Empire seem to rarely have engaged in mixed marriages with non-Greeks; they kept to their own cities.

The various non-Greek peoples of the empire were still influenced by the spread of Greek thought and culture, a phenomenon referred to as Hellenization. Historically significant towns and cities, such as Antioch, were created or renamed with Greek names, and hundreds of new cities were established for trade purposes and built in Greek style from the start. Local educated elites who needed to work with the government learned the Greek language, wrote in Greek, absorbed Greek philosophical ideas, and took on Greek names; some of these practices then slowly filtered down to the lower classes. Hellenic ideas began an almost 250-year expansion into the Near East, Middle East, and Central Asian cultures.

Synthesizing Hellenic and indigenous cultural, religious, and philosophical ideas – an ethnic unity framework established by Alexander – met with varying degrees of success. The result was times of simultaneous peace and rebellion in various parts of the empire. In general, the Seleucids allowed local religions to operate undisturbed, such as incorporating Babylonian religious tenets, to gain support. Tensions around the integration of Judaism were present during the reign of the Seleucid governments. Though previous governments had managed a relatively seamless integration of Judean religious and cultural practices, the rule of Antiochus IV introduced significant changes. Antiochus IV instigated a bidding process for the High Priest position—this led to Menelaus, a radical Hell…

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As with the other major Hellenistic armies, the Seleucid army fought primarily in the Greco-Macedonian style, with its main body being the phalanx. The phalanx was a large, dense formation of men armed with small shields and a long pike called the sarissa. This form of fighting had been developed by the Macedonian army in the reign of Philip II of Macedon and his son Alexander the Great. Alongside the phalanx, the Seleucid armies used numerous native and mercenary troops to supplement their Greek forces, which were limited due to the distance from the Seleucid rulers' Macedonian homeland. The size of the Seleucid army usually varied between 70,000 and 200,000 in manpower.

The distance from Greece put a strain on the Seleucid military system, as it was primarily based around the recruitment of Greeks as the key segment of the army. In order to increase the population of Greeks in their kingdom, the Seleucid rulers created military settlements. There were two main periods in the establishment of settlements, firstly under Seleucus I Nicator and Antiochus I Soter and then under Antiochus IV Epiphanes. The military settlers were given land, "varying in size according to rank and arm of service'. They were settled in 'colonies of an urban character, which at some point could acquire the status of a polis". The settler-soldiers were called katoikoi; they would maintain the land as their own and in return, they would serve in the Seleucid army when called. The majority of settlements were concentrated in Lydia, northern Syria, the upper Euphrates and Media. Antiochus III brought Greeks from Euboea, Crete and Aetolia and settled them in Antioch.

These Greek settlers would be used to form the Seleucid phalanx and cavalry units, with picked men put into the kingdom's guards' regiments. The rest of the Seleucid army would consist of native and mercenary troops, who would serve as light auxiliary troops. While the Seleucids were happy to recruit from less populated and outlying parts of the Empire such as the Arabs and Jews, Iranian peoples in the east, and inhabitants of Asia Minor to the north, they generally eschewed recruiting native Syrians and native Mesopotamians (Babylonians). This was presumably mostly from a desire not to train and arm the people who were an overwhelming majority in the trade and governmental centers of the Empire in Antioch and Babylon, risking revolt. While a revolt in a remote place could be put down by resolute action from the center, an uprising in Syria-Coele would have undermined the kingdom's very existence.

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 en…

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