The Parthians drive back the last Seleucid…
129 BCE
The Parthians drive back the last Seleucid counterattack in 129 BCE.
The loss of so much territory sends the already enfeebled empire into a decline from which it can never recover.
The Seleucid Empire becomes a rump state that consists of little more than Antioch and the surrounding lands.
The only reason the Seleucid Empire continues to exist is because the Parthians see it as a useful buffer against the expansionist Roman Republic.
When Pompey leads a Roman expedition into Syria, in 63 BCE, he will annex the Seleucid Empire, and the stage will be set for the Roman–Parthian Wars.