When Alexander the Great died in 323 BC, his general Seleucus became king of most of the former Persian Empire. Around 250 BC the Parthians, led by Arsaces I, seceded from the Seleucid Empire.
When the Seleucid Empire began to crumble after 190 BC, the Parthians, led by Mithridates I, seized the opportunity to conquer regions to their east, west and south, as far as the Zagros mountains.
After further dramatic conquests, Mithridates captured the Seleucid capital, Seleucia-on-the-Tigris, in 141 BC and gained control of the rich Mesopotamian and Babylonian plains. He built a new capital, Ctesiphon, opposite Seleucia.
Some eastern territories were lost to the Shakas in 135 BC and western areas to the Romans in 64 BC. The Euphrates river became the rough boundary between the rival Parthian and Roman Empires.
For 200 years Parthia and Rome fought for control of the Euphrates River, vitally important as a trade route. Nevertheless Parthia's borders changed little until it was overthrown by the Sassanians in AD 226.
