The siege of Orchomenes (418 B.C.) was a short-lived success won by an alliance of Greek cities led by Argos and that included Athens. In 421 BC the Peace of Nicias had temporarily ended the fighting during the Great Peloponnesian War. One of Sparta's reasons for agreeing to the peace was that their peace treaty with Argos, a key rival in the Peloponnese, was about to expire and Sparta didn't want a war on two fronts.
The peace of Nicias was unpopular in Corinth, another of Sparta's allies. The Corinthians convinced the Argives to form a new defensive alliance, aimed at reducing the power of Sparta in the Peloponnese. Over the next two years this new alliance evolved until by 418 BC it included Athens, Mantinea and the Eleans, as well as Argos. Ironically Sparta could still call on Corinth, where enthusiasm for the new alliance had quickly faded. The Boeotians also remained loyal, despite having briefly dallied with the Argive alliance.
In the summer of 418 BC the Argives attacked the Epidaurians in Arcadia. The Spartans moved to defend their allies, and summoned their own allies. Argos also summoned their allies, and a battle seemed inevitable. Instead, as the two armies faced each other outside the city of Argos their leaders agreed not to fight. The Spartans then pulled back across their borders.
Soon after this the Athenians arrived, and convinced their allies to keep fighting. Their first move was to attack the city of Orchomenos in Arcadia, where the Spartans were holding some hostages. The allied army arrived outside the city, besieged it, and carried out a number of unsuccessful assaults on the place.
Despite these failures, the defenders were worried. Their defences were weak and their allies weren't close enough to arrive in time. They decided to surrender on terms - they were to change sides, join the Argive alliance, free the Spartan hostages and provide hostages of their own.
After the surrender of Orchomenos the allies decided to attack Tegea, a key Spartan ally. They moved to Mantinea to prepare for the attack, but instead suffered a heavy defeat at the hands of the Spartans at the battle of Mantinea.