After the Parthian Kingdom was founded, the Seleucid Empire was weakened by internal struggles and the difficulty of ruling far eastern provinces. Mithridates I aimed to secure richer lands and the main routes where people and goods moved in the west, so he began a long campaign against Seleucid governors.
Starting in 171 BC, Parthian forces advanced west and first seized Media. About six years later, in 141 BC, they pushed further to capture Seleucia on the Tigris, a major administrative center. This victory shifted the balance of power, giving Parthia control over the heart of Mesopotamia.
Twelve years after the fall of Seleucia, in 129 BC, the Seleucid Empire launched a final massive counterattack under Antiochus VII to regain their lost eastern territories. However, he was defeated and killed at the Battle of Ecbatana . This decisive defeat ended Seleucid influence in the east forever and established Parthia as a major empire capable of challenging Rome. Following this, the Seleucid Empire was reduced to a minor power confined to Syria, plagued by civil wars until it was finally annexed by Rome in 63 BC.