History of the Peloponnesian War

The Argives had been aware of the preparations of the Lacedaemonians from the first, and when the latter were on the march to Phlius where they intended to join the rest, they now took the field themselves. And the Mantineans came to their aid with their own allies and three thousand Elean hoplites.

As they were going forward they came upon the Lacedaemonians at Methydrium in Arcadia. Each party took position on a hill, and the Argives prepared to fight with the Lacedaemonians, thinking to find them still isolated; but Agis, rousing up his force during the night and eluding detection, marched to Phlius to join the rest of the allies.