Around 426 or 425 BCE, an army from the Peloponnesian League set out to conquer Amphilochian Argos, a city founded on the southeast end of the Ambracian Gulf, and not to be confused with the more widely known Argos in the Peloponnesus. This army, according to Thucydides (c. 460-400 BCE), had around 3,000 heavy infantry hoplites and was commanded by a Spartan general named Eurylochus. In addition to this, a further 3,000 hoplites invaded Amphilochian Argos from pro-Peloponnesian Ambracia, located just north of the Ambracian Gulf. This Ambraciot army was the faster of the two invading forces, so they seized the stronghold of Olpae without Peloponnesian help, and in doing so, gained a strong position just a few miles from Amphilochian Argos.
Eurylochus and the Peloponnesian army apparently did not launch their invasion until after the Ambraciots had already seized Olpae. As a result, the Amphilochians had time to muster their manpower and call for Athenian help. As the Ambraciots waited for Eurylochus’ army, the Amphilochians reinforced their city of Argos and placed scouts at a region called Crenae, in order to watch for the Peloponnesian army. They also successfully contacted, Demosthenes, one of Athens’ craftiest generals—he answered their plea and arrived with twenty ships. His fleet was only carrying a reported 200 hoplites and 60 archers, but Demosthenes’ knack for odd strategies would make up for the lack of numbers. In addition, the Amphilochians also received military aid from Acarnania, a neighboring region that survived a Peloponnesian invasion between 429 and 428 BCE. Demosthenes was appointed as commander-in-chief of this coalition of forces and he marched their united front to challenge the Ambraciots at Olpae.