During classes about the peleponisian war we hear that Argos was often at war with it's neighbour Sparta. Later on we also see that it has joined in an alliance with Athens.
So what happens to Argos during the peleponisian war? What was their role and what did Athens defeat mean to them?
The Argives were neutral during the early part of the war, but after the Peace of Nicias in 421 they allied themselves with the Athenians. The Argives had been allied with the Athenians, a fellow democracy, in 461, and bore a long-standing grudge against the Spartans, with whom they had fought bitterly during the Archaic Period--the OCD says, kind of amusingly, that the Argives maintained a "suspicious neutrality" towards the Spartans, fighting briefly with them about once a generation. The Athenians had used the Peace of Nicias to conclude alliances with several Peloponnesian states, notably Argos, Elis, and Mantinea. In 418 the Spartans marched out under Agis to seize Tegea, which was plotting to join the Argives, and the allies mustered near Mantinea. The army, however, was composed mostly of Argives, with only a thousand Athenians. The Athenians were still technically at peace with the Spartans, as the Peace of Nicias was not officially abandoned until 414 (although it was pretty much ignored from the start), and could not risk a large force, so the fighting was left mostly to the Argives, who hadn't fought a major battle in decades. Thucydides says that while the Argive right, composed of the Mantineans and 1000 picked Argives successfully routed the enemy's flank, which had fallen into disarray due to issues with insubordination among the Spartan commanders, the Argive center, composed mostly of old men, routed instantly. Thucydides accuses most of the Argives of fleeing before the Spartans even closed, with such panic that they trampled some of their own men. Thucydides describes the allied army as cut in two, with the Athenians on the left and the Mantineans on the right both holding their own, but the Athenians slowly being encircled--Thucydides says that Agis deployed his troops to assist on his left, where the Mantineans had broken completely through, and the Athenians used this opportunity to screen their forces with their cavalry and escape in good order, whereas the Mantineans were counterattacked and put to flight.
Following Mantinea the Argives concluded a peace with the Spartans, renouncing their alliances with Athens and Elis. The peace resulted in an oligarchic coup in the city, and Argos didn't really do much until the Corinthian War