Sparta’s constitution took on the form it would have in the Classical period during the eighth century BC. By the classical period, Spartan tradition attributed this constitution to Lycurgus of Sparta, which was dated by Thucydides to a little over four centuries before the end of the Peloponnesian War, or around the end of the ninth century. The First Messenian War, probably taking place from approximately 740 to 720 BC, saw the strengthening of the powers of the Gerousia against the assembly, and the enslavement of the Messenian population as Helots. Around the same time, the ephors gained the power to restrict the actions of the kings of Sparta. Thus by the late seventh century, Sparta’s constitution had recognisably taken on its classical form.
From around 560 BC, Sparta began to build a series of alliances with other Greek states, which became the Peloponnesian League: by 550, cities such as Elis, Corinth, and Megara would be part of the alliance. This series of alliances had the dual purpose of preventing the cities of the League from supporting the Helot population of Messenia, and of helping Sparta in its conflict with Argos, which in the archaic period was along with Sparta one of the major powers in the Peloponnese.
Source: Wikipedia
