Seleucid power had begun to weaken after the defeat of Antiochus the Great at the hands of the Romans at the Battle of Magnesia in 190 BCE effectively broke Seleucid power and in particular the Seleucid army.
The Arsacids had taken power in Parthia and declared their full independence from the Seleucid Empire.
The Parthian king Mithridates I of Parthia invades Media, which is already in revolt against the Seleucid empire, in 148 BCE.
After winning a decisive battle at the Battle of Ecbatana, Mithridates is able to annex Media from the Seleucid Empire.
