The Delian League was an association of Greek city-states in the 5th century BC. Because many of the league's poleis were too poor to contribute ships to the collective navy, they paid their phoros (membership dues) to Athens in the form of money, so that there would be enough money to build the expensive triremes.
In 478 BC, following the defeat of Xerxes' invasion of Greece, Pausanias the Spartan led Hellenic forces against the Persians. He was an unpopular commander (who may have conspired with the Persians), and although he was cleared of all accusations of conspiracy, Sparta, eager to stop prosecuting the war, decided to remain outside the war against Persia. Spartans were of the view that with the liberation of the Greek cities in Asia Minor, the war's purpose had already been reached; in this wise being opposed to the Athenians, who felt related to the Ionian Greeks, and wanted more than to free them: they wanted to continue the war in order to provide security to the Greeks in Asia Minor. In this way, Sparta surrendered the leadership of the ongoing campaign to Athens, which was eager to accept it. The Delian League by the military actions of the Athenians. The justification for this was that Carystus was enjoying the advantages of the League (protection from pirates and the Persians) without taking on any of the responsibilities. Furthermore, Carystus was a traditional base for Persian occupations. Athenian politicians had to justify these acts to Athenian voters in order to get votes. The island of Naxos, a member of the Delian League, attempted to secede, and was enslaved; Naxos is believed to have been forced to tear down its walls, lose its fleet and its vote in the League.
Thucydides tells us that this is how Athens' control over the League grew.
In 454 BC, Athens moved the treasury of the Delian League from Delos to Athens, allegedly to keep it safe from Persia. However, Plutarch indicates that many of Pericles' rivals viewed the transfer to Athens as usurping monetary resources to fund elaborate building projects. Athens also switched from accepting ships, men and weapons, to only accepting money. The new treasury established in Athens was used for many purposes, not all relating to the defense of members of the league. It was from tribute paid to the league that Pericles sent to build the Parthenon in the acropolis, as well as many other non-defense related expenditures. Some claim that during this time the Athenian Empire arose, as the technical definition of empire is a group of cities paying taxes to a central, dominant city, while keeping local governments intact. The Delian League was turning from an alliance into an empire.
In 465 BC Thasos revolted against the Delian League. After two years Thasos surrendered to Cimon. In result, the fortification walls of Thasos were torn down, their land and naval ships were confiscated by Athens. The mines of Thasos were also turned over to Athens and they had to pay yearly tribute and fines.
In 461 BC, Cimon was ostracized, and was succeeded in his influence by democrats such as Ephialtes and Pericles. This signaled a complete change in Athenian foreign policy, neglecting the alliance with the Spartans and instead allying with her enemies, Argos and Thessaly. Megara deserted the Peloponnesian league and allied herself with Athens, allowing construction of a double line of walls across the isthmus of Corinth, protecting Athens from attack from that quarter. Around the same time, due to encouragement from influential speaker Themistocles, they also constructed the Long Walls connecting their city to the Piraeus, its port, making it effectively invulnerable to attack by land.
Soon war with the Peloponnesians broke out. In 458 BC, the Athenians blockaded the island of Aegina, and simultaneously defended Megara from the Corinthians by sending out an army composed of those too young or old for regular military service. The next year Sparta sent an army into Boeotia, reviving the power of Thebes to help hold the Athenians in check. Their return was blocked, and they resolved to march on Athens, where the Long Walls were not yet completed, winning a victory at the Battle of Tanagra. All this accomplished, however, was to allow them to return home via the Megarid. Two months later, the Athenians under Myronides invaded Boeotia, and winning the battle of Oenophyta gained control of the whole country except Thebes.
War with the Persians continued, however. In 460 BC, Egypt had revolted under Inarus and Amyrtaeus, who requested aid from Athens. Pericles led 250 ships, originally intended to attack Cyprus, to their aid because it would hurt Persia. Persia's image had already been hurt when it failed to conquer the Greeks and Pericles wanted to further this. After four years, however, the rebellion was defeated by general Megabyzus, who captured the greater part of the Athenian forces. In fact, according to Isocrates, the Athenians and their allies lost some 20,000 men in the expedition. The remainder escaped to Cyrene and thence returned home.
This was Athenians' main (public) reason for moving the treasury of the League from Delos to Athens, further consolidating their control over the League. The Persians followed up their victory by sending a fleet to re-establish their control over Cyprus, and 200 ships were sent out to counter them under Cimon, who returned from ostracism in 451 BC. He died during the blockade of Citium, though the fleet won a double victory by land and sea over the Persians off Salamis.
This battle was the last major one fought against the Persians. Many writers report that a formal peace treaty, known as the Peace of Callias, was formalized in 450 BC, but some writers believe that the treaty was a myth created later to inflate the stature of Athens. However, an understanding was definitely reached, enabling the Athenians to focus their attention on events in Greece proper.
The peace with Persia, however, was followed by further reverses. The Battle of Coronea, in 447 BC, led to the abandonment of Boeotia. Euboea and Megara both revolted, and while the former was restored to its status as a tributary ally, the latter was a permanent loss. The Delian and Peloponnesian Leagues signed a peace treaty, which was set to endure for thirty years. It only lasted until 431 BC, when the Peloponnesian War broke out.
Those who revolted unsuccessfully during the war saw the example made of the Mytilenians, the principal people on Lesbos. After an unsuccessful revolt, the Athenians ordered the death of the entire male population. After some thought, they rescinded this order, and only put to death the leading 1000 ringleaders of the revolt, and redistributed the land of the entire island to Athenian shareholders, who were sent out to reside on Lesbos.
This type of treatment was not reserved solely for those who revolted. Thucydides documents the example of Melos, a small island, neutral in the war, though originally founded by Spartans. The Melians were offered a choice to join the Athenians, or be conquered. Choosing to resist, their town was besieged and conquered; the males were put to death and the women sold into slavery (see Melian dialogue).
The Delian League was never formally turned into the Athenian Empire; but by the start of the Peloponnesian War, only Chios and Lesbos were left to contribute ships, and these states were by now far too weak to secede without support. Lesbos tried to revolt first, and failed completely. Chios, the greatest and most powerful of the original members of the Delian League (save Athens), was the last to revolt, and in the aftermath of the Syracusan Expedition enjoyed a success of several years, inspiring all of Ionia to revolt. Athens was, however, still able to eventually suppress these revolts.
The Athenian Empire was very stable, and only 27 years of war, aided by the Persians and internal strife, were able to defeat it. The Athenian Empire did not stay defeated for long. The Second Athenian Empire, a maritime self-defense league, was founded in 377 BC and was led by Athens; but Athens would never recover the full extent of her power, and her enemies were now far stronger and more varied.