Alexander the Great had conquered the Persian Empire within a short time-frame and died young, leaving an expansive empire of partly Hellenized culture without an adult heir. Therefore his generals (the Diadochi) jostled for supremacy over portions of his empire.
Seleucus, one of his generals, established himself in Babylon in 312 BC, used as the foundation date of the Seleucid Empire. He ruled over not only Babylonia, but the entire enormous eastern part of Alexander's Empire. Following his and Lysimachus' victory over Antigonus Monophthalmus at the Battle of Ipsus in 301 BC, Seleucus took control over eastern Anatolia and northern Syria. In the latter area he founded a new capital at Antioch on the Orontes, a city he named after his father. An alternative capital was established at Seleucia on the Tigris, north of Babylon. Seleucus' empire reached its greatest extent following his defeat of his erstwhile ally, Lysimachus, at Corupedion in 281 BC. Seleucus expanded his control to encompass western Anatolia. He hoped further to take control of Lysimachus' lands in Europe - primarily Thrace and even Macedonia itself, but was assassinated by Ptolemy Ceraunus on landing in Europe. His son and successor, Antiochus I Soter, proved unable to pick up where his father had left off in conquering the European portions of Alexander's empire, but was left, nevertheless, with an enormous realm consisting of nearly all of the Asian portions of the Empire. His competitors were Antigonus II Gonatas in Macedonia and Ptolemy II Philadelphus in Egypt.
The Seleucid empire's geographic span, from the Aegean Sea to Afghanistan, brought together a multitude of races: Greeks, Persians, Medes, Jews, Indians, to mention only some. Its rulers were in the position of having a governing interest to implement a policy of racial unity initiated by Alexander. By 313 BC, Hellenic ideas (disseminated by the conquering Macedonian army's hired philosophers and historians, retired officers, and married inter-racial couples) had begun their almost 250-year expansion into the Near East, Middle East, and Central Asian cultures. It was the empire's governmental framework to rule by establishing hundreds of cities for trade and occupational purposes. Many of the existing cities began -- or were compelled by force -- to adopt Hellenized philosophic thought, religious sentiments, and politics. Synthesizing Hellenic and indigenous cultural, religious, and philosophical ideas met with varying degrees of success -- resulting in times of simultaneous peace and rebellion in various parts of the empire.
Nevertheless, even before Seleucus' death, the vast eastern domains of the Seleucids were proving difficult to assert control over. Seleucus had invaded India (modern Punjab in northern India and Pakistan) in 304 BC, leading to conflict with the Mauryan empire ruled by Chandragupta Maurya (Sandrokottos). It is said that Chandragupta fielded an army of 100,000 men and 9,000 war elephants, and forced Seleucus to conclude an alliance and give him his daughter in marriage. In exchange Chandragupta gave him no less than 500 elephants, an addition to his army that was to play a prominent part in his victory at Ipsus.
Seleucus sent an ambassador named Megasthenes to Chandragupta's court, who repeatedly visited Pataliputra (modern Patna in Bihar state), capital of Chandragupta. Megasthenes wrote detailed descriptions of India and Chandragupta's reign, which have been partly preserved to us through Diodorus Siculus.
Other territories lost before Seleucus' death were Gedrosia in the south-east of the Iranian plateau, and, to the north of this, Arachosia on the west bank of the Indus River. Antiochus I (reigned 281-261 BC) and his son and successor Antiochus II Theos (reigned 261-246 BC) were faced with challenges in the west, including repeated wars with Ptolemy II and a Celtic invasion of Asia Minor -- distracting attention from holding the eastern portions of the Empire together. Towards the end of Antiochus II's reign, the eastern provinces of Bactria and Parthia simultaneously asserted their independence.
By the time Antiochus II's son Seleucus II Callinicus came to the throne around 246 BC, the Seleucids seemed to be at a low ebb indeed. Aside from the secessions of Parthia and Bactria, Seleucus II was soon dramatically defeated in the Third Syrian War against Ptolemy III of Egypt, then had to fight a civil war against his own brother Antiochus Hierax. In Asia Minor too, the Seleucid dynasty seemed to be losing control -- Gauls had fully established themselves in Galatia, semi-independent semi-Hellenized kingdoms had sprung up in Bithynia, Pontus, and Cappadocia, and the city of Pergamum in the west was asserting its independence under the Attalid Dynasty.
But a revival would begin when Seleucus II's younger son, Antiochus III the Great, took the throne in 223 BC. Although initially unsuccessful in the Fourth Syrian War against Egypt, which led to an embarrassing defeat at the Battle of Raphia (217 BC), Antiochus would prove himself to be the greatest of the Seleucid rulers after Seleucus I himself. Following his defeat at Raphia, he spent the next ten years on his Anabasis through the eastern parts of his domain -- restoring rebellious vassals like Parthia and Bactria to at least nominal obedience, and even emulating Alexander with an expedition into India.
When he returned to the west in 205 BC, Antiochus found that with the death of Ptolemy IV, the situation now looked propitious for another western campign.
Antiochus and Philip V of Macedon then made a compact to divide the Ptolemaic possessions outside of Egypt, and in the Fifth Syrian War, the Seleucids ousted Ptolemy V from control of Coele-Syria. The Battle of Panium (198 BC) definitively transferred these holdings from the Ptolemies to the Seleucids. Antiochus appeared, at the least, to have restored the Seleucid Kingdom to glory.
