The Delian League was an association of Greek city-states in the 5th century BC. As it was led by Athens, it is sometimes perjoratively referred to as the Athenian Empire.
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 Sparta was eager to stop prosecuting the war. They surrendered the leadership of the ongoing campaign to Athens, which was eager to accept it. The Delian League was inaugurated in 477 BC as an offensive and defensive alliance against Persia. The principal cities in the League were Athens, Chios, Samos, and Lesbos, but many of the principal islands and Ionian cities joined the league.
Athens led the Delian League from the beginning, though at its founding the treasury was located on the island of Delos, and each state in the league had an equal vote. The assessment due from each state was assigned by Aristides the Just, leader of the Athenians; some members were assessed ships, others money. A council of all the cities met at Delos regularly, probably when bringing their assessment to the island.
The first action of the Delian League, under the command of Cimon, was the capture of Eion, a Persian fortification that guarded a river crossing on the way to Asia; following this victory, the League acted against several pirate islands in the Aegean Sea, most notably against Scyrus where they turned the Dolopian inhabitants into slaves and set up a cleruchy. A few years later they sailed against Caria and Lycia, defeating both the Persian army and navy in the battle of the Eurymedon.
These actions were most likely very popular with the League. However, the League, particuarly the Athenians, were willing to force cities to join the League. Carystus, a city on the southern tip of Euboea, was forced to join the League by 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. Naxos, a member of the Delian League, attempted to secede, and was enslaved; Naxos is believed to have been forced to tear down her walls, lost her fleet, and her vote in the League.
Thucydides tells us that this is how Athens control over the League grew.
Of all the causes of defection, that connected with arrears of tribute and vessels, and with failure of sevice, was the chief; for the Athenians were very severe and exacting, and made themselves offensive by applying the screw of necessity to men who were not used to and in fact not disposed for any continuous labour. In some other respects the Athenians were not the old popular rulers they had been at first; and if they had more than their fair share of service, it was correspondingly easy for them to reduce any that tried to leave the confederacy. For this the allies had themselves to blame, the wish to get off service making most of them arrange to pay their share of the expense in money instead of in ships, and so to avoid having to leave their homes. Thus while Athens was increasing her navy with the funds they contributed, a revolt always found them without resources or experience for war. [Thucydides i. 99]
In 461 BC, Cimon was ostracized, and was succeeded in his influence by democrats like Ephialtes and Pericles. This signalled 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 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 200 ships, originally intended to attack Cyprus, to their aid. After four years, however, the rebellion was defeated by the general Megabyzus, who captured the greater part of the Athenian forces. The remainder escaped to Cyrene and thence returned home.
Fearing retribution for all this, the Athenians moved 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 Island.
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 formalised 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 recinded 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.