At the time of the Peloponnesian War, from 431 to 404 B.C., the Greek (Hellenic) world extended across Greece proper, Albania, Macedonia, Southern Italy, Sicily, Ionia (Western part of Anatolia), Western Asia Minor, Byzantium (the name of Constantinople in that period), and parts of France, Spain and North Africa. Since the city-states that were part of this Greek world were modeled on the wider Hellenic culture, which had evolved during the Homeric Age, there was an extensive social and economic ecology between them, and this vast area can be seen as a loose federation or union of Hellenic city-states.
The Peloponnesian War was not a struggle for hegemony between just Athens and Sparta. The entire Greek world went to war. Every city state was either with the Delian League led by Athens, or the Peloponnesian League led by Sparta. Neutrality was not an option. Athens was a sea power and Sparta was a land power. But many city-states that were fighting with Sparta were sea powers and this ensured that Peloponnesian League was maintaining a considerable fleet of triremes (Greek war galley) throughout the conflict. In the Delian League, there were a number of city-states which were land powers which owned powerful hoplite armies.
The major Greek city-states sided with Sparta, which at the time of the outbreak of the Peloponnesian War, saw itself as the cultural and economic leader of the Greek world. The Greeks, including the Athenians, admired the Spartans for their rule of law and their virtues. The Spartans brought peace and stability to the Greek world—they protected the trade routes, and facilitated commerce between the city-states. In the final phase of the war, King Darius II of the Persian Achaemenid Empire entered the conflict on the side of the Spartans. The extensive economic relationship that the Spartan side had with the Persians would have influenced the decision of Darius II to support the Spartans.
The Spartans and the Athenians had fought together when the Persian Kings Darius I and Xerxes attempted to conquer the Greek world. It was primarily the 10000 strong Athenian hoplite army which defeated King Darius I in the Battle of Marathon in 490 B.C. Though Spartan King Leonidas and his 300 Spartans attained immortality by fighting to death against Xerxes, they could not stop the Persian army from entering Greece and sacking Athens. The Athenians survived because they had left their city, having been convinced by their leader Themistocles that they must live to fight another day. They decisively defeated Xerxes at the Battle of Salamis in 480 B.C. Xerxes was humiliated by the defeat. He was forced to return to Persia to secure his throne.
Themistocles and the Athenians were hailed as the saviors of the Hellenes. By 477 B.C., the Athenians had established their own hegemony. The Spartans were surprised to see that the Athenians had not only a powerful fleet but also powerful alliances. Themistocles stated in an Athenian assembly that Sparta was a greater foe than Persia. Most Athenians disagreed, but the Spartans were incensed. Under Spartan pressure, Themistocles was ostracized by the Athenian assembly in 476 B.C., and in 472 B.C. he was exiled to Argos.
Cimon (son of Miltiades) became the new leader of the Athenians. He was an able general. In the 470s, he evicted the Persian navy from the region that is today known as the Dardanelles or the Bosporus. He destroyed the Persian contingents stationed in Northern Greece and brought the city-states there into the Delian League. In the opinion of Thucydides, the Athenians started asserting themselves throughout the Greek world after their success in the two Persian Wars and this drove the Spartans and their allies to declare war on Athens in 431 B.C.
There were a number of clashes between the Athenians and the Spartans in the 460s B.C. In 461 B.C., when the fate of Athens went into the hands of Pericles and his radicalized assembly, a point of no return was quickly reached, and Athens and Sparta embarked on the First Peloponnesian War. This war ended with a thirty year peace treaty in which Athens had to give up most of its possessions in the Greek mainland which it had acquired since 460 B.C. But several small wars took place during the period of peace. In 432 B.C., King Archidamus II of Sparta warned the Spartans that the coming war with Athens would be long and brutal. Pericles tried to peacefully settle the disputes in the Hellenic world but he died in 429 B.C.
By 425 B.C., the league led by the Spartans and the league led by the Athenians were locked in mortal combat in which neither decisive victory nor a negotiated settlement was possible. Victory would go to the side which had the resources, patience, and the resolve to fight till the end. The Spartan side had these qualities and they won in 404 B.C.