Seleucid Empire

One of the Diadochoi, or Alexander's successor-states, also the largest. At it's height, the Seleucids cevered everything in between modern Turkey, Armenia, Syria, Arabia, Kazakhstan and Iran. It was founded by Seleukos Syriakos I Nikator in 312BCE with his capital, Antioch-on-the-Orontes (named after his Father). After extablishing his power in Mesopotamia and Syria, he re-conquered the lands of Alexander to the East until the Indus River; there, instead of the squabbling little Indian kingdoms he expected to find, was a large kingdom which rules over the whole of Northern India known as the Mauryan Empire ruled by Chandragupta Maurya. After warring with Chandragputa for a few years, Seleukos made a peace treaty with him and exchanged a part of the Seleucid empire for 500 war elephants which would serve him well in the battles against Ptolemaios I Soter, Antigonos I Monophthtalmos and Lysimachos, mainly at the Battle of Ipsos.
Circa 245BCE, Basileis Diodotos asserted his independence from the Seleucids in his kingdom of Baktria. The Kingdom of Baktria was to last until 125BCE during the invasion of the Yuezhi Nomads from Mongolia and in 180BCE, the Baktrian Basileis Demetrios I lead an army into the Indus Valley and founded the Graeco-Indian Kingdom.
in 246 BCE, Seleukos II Kallinikos waged the Third Syrian War against the Greek Pharoas of Aigyptos and defeated Ptolemaios III but was defeated. Seleukos also had to fight a civil war against his brother Antiochos Herax. With this distraction for Seleukos, Baktria and the Arsacids (Parthians) seceded from the Seleucid empire. In Bythinia, Pontos and Kappadocia too, were rebellions cropping up everywhere with the Galatian Celts, Pontic Kingdom and the Arkah of Hayasdan. After the Reign of Seleukos II, came the reign of Antiochos III, whom depite his embarassing defeat at Raphia in 217 BCE, managed to re-vassalize the Baktrians. Under the advice of the exiled ex-Carthaginian Shophet Hannibal Barca his military advisor, Antiochos III launched the invasion of Greece. The Romans fearing that Antiochos would gain too much power, decided to interfere (as usual as what happened to the Makedonians and the Carthaginians). he was defeated by the Roman Legions at Thermopylae and Magnesia and was forced to surrender all of his territory in Europe and Anatolia.
After Antiochos III's reign, came that of Seleukos IV Philopator and Antiochos IV Epiphanes. the Seleucids began the slow downfall right after Antiochos IV's attempts to hellenize the empire by forcing Jews and Persians to adopt Greek customs and gods but that caused massive civil unrest together with the Parthian invasion of former Persian lands by Mithradates II.
In 129BCE, the death of Antiochos VII lead to the loss of Mesopotamia, the Iranian Plateu and Civil Wars. by 100BCE, the empire comprised of only Antioch and some Syrian cities and in 83BCE, Tigranes the Great, Arkah of Hayasdan, invaded Syria and effectively ending Seleucid rule.