The Seleucid Empire was the Persian kingdom of the Macedonian dynasty of the Seleucids, whose rule began with the collapse of Alexander's empire and faded away between Roman and Parthian growth of power in the 1st century BCE.
The Seleucid Empire began when Seleucos I, one of Alexander the Great's former favorite companions, was given the satrapy of Babylon in the second division of the empire in 321 BCE. He first ruled it briefly until 315 BCE, when he was forced to flee to Egypt under pressure of Antigonos. There he prepared his revenge with the help of Ptolemy, and succeeded to retake Babylon after the battle of Gaza in 305 BCE. He also inherited the Asian part of Antigonos' vast empire after its final fall at the battle of Ipsos in 301 BCE. Having secured Antigonos' kingdom's eastern part, Seleucos managed to reconquer most of Alexander's empire, defeating Lysimachos and Demetrios. He was, however, murdered in 281 BCE on the eve of his success by the man he supported on the Egyptian throne, Ptolemy Keraunos.
After the death of Seleucos, things became worse for his successors. During the successive reigns of Antiochos I, Antiochos II, Seleucos II and Seleucos III, the empire struggled, due to rebellions of Bythinia, Pergammum, Bactria and Parthia, and the first indecisive Syrian wars against the Ptolemies. Internal struggles began during this time, which continued until the empire's end. The Seleucids also had to fight the Galatians who devastated Anatolia, and also against rebellious elements at all levels.
It is this disorganized and problematic empire that the eighteen year-old Antiochos III inherited in 223 BCE. Over the next 25 years he subdued most of the rebellious states in a great tour de force: He made his anabasis (difficult retreat) in the east successfully fighting Parthians and Bactrians, made a profitable treaty with the Indian ruler Sophagasenos and confirmed his superiority on rebellious subjects. He also made an expedition against the Gerrhaeans of the East Arabian coast in 204 BCE and defeated the Ptolemies twice which allowed him to take control of the highly valued Koile Syria near 198 BCE.
Regrettably, he also led a war against Rome in the wake of his expansion in Anatolia, and despite the wise advice of the Carthaginian Hannibal Barca, which he decided not to follow, he was defeated at the Battle of Magnesia ad Sipylum in 190 BCE. The consequences of the disastrous peace treaty which followed led the kingdom into ruin, and Antiochos III died in 187 BCE during a campaign in the East.
Antiochos III's death marked the end of the Seleucid Empire as a great power. The kingdom fell once more into dynastic struggles, and the eastern provinces were gradually lost due to rebellions and Parthian expansion. Much worse was the Roman interference in the Empire, largely influencing the dynastic quarrels and foreign policy, such as in 168 BCE when the Romans forced Antiochos IV to withdraw from the only successful Seleucid campaign in Egypt. The wild intrigues which characterized the last decades of the Seleucid Empire were ended by the invasion of the Armenian king Tigranes II in 83 BCE. Even if after Tigranes some rulers of Syria claimed to be Seleucid kings, they were no more than Roman vassals.
The Seleucid legacy in Asia was strong, because Hellenism was established in Asia during two centuries of Seleucid rule. The method of dating years in Asia, for example, was called the Seleucid Era, beginning at the return of Seleucos I to Babylon in 311 BCE, which was continued to be used as late as the 6th century CE. In fact, the Seleucid legacy lasted throughout Roman, Parthian and Sassanid dominion until the Arabian invasions of the 7th century CE introduced Islam.