Xerxes returns to Persia after the military blunders in Greece, and completes the many construction projects left unfinished by his father at Susa and Persepolis.

He builds the Gate of all Nations and the Hall of a Hundred Columns at Persepolis, which are the largest and most imposing structures of the palace.

He completes the Apadana, the Palace of Darius and the Treasury all started by Darius as well as building his own palace which is twice the size of his father's.

His taste in architecture is similar to that of Darius, though on an even more gigantic scale.

He also maintains the Royal Road built by his father, …

…completes the Susa Gate and builds a palace at Susa.

Sataspes, a Persian navigator and cavalry commander—who, according to Herodotus, was Xerxes I's cousin by his mother being Darius I's sister—had been condemned to death for kidnapping and raping Megabyzus's daughter.

However his mother, Atossa, had successfully persuaded Xerxes to change the punishment to a more severe one—Sataspes was tasked to circumnavigate Africa.

He took an Egyptian ship and crew, sailed through the Pillars of Hercules, and proceeded south for many months, but returned to Egypt without successfully completing his task.

He claimed that at the farthest point he reached, he encountered a "dwarfish race, who wore a dress made from the palm tree", and that he was forced to return because his ship stopped and would not sail any further.

Xerxes did not accept this excuse, and had him put to death.

However, it has been suggested that Sataspes could have simply encountered the Benguela Current, the broad, northward flowing ocean current off southwest Africa, which prevented him from sailing any farther.

Of the Ionian cities, Ephesus maintains friendly relations with Persia: in 478, Xerxes, returning from his failure in Greece, honors Artemis of Ephesus, although he sacks other Ionian shrines, and leaves his children for safety in Ephesus.

Miletus, after the defeat of the Persian invaders in 479 BCE by the Greeks, joins the Athenian-dominated Delian League.

Spartan leadership had gone unquestioned by Athens in the Persian Wars of 480-479, except for one or two uneasy moments when it had seemed that Sparta was reluctant to go north of the Isthmus.

In one manifestation of Athenian independence from Sparta, Athens in early 478 on its own account, assisted by “allies from Ionia and the Hellespont,” captures Sestus, hitherto still under precarious Persian control.

The victorious Greeks subsequently punish Thebes by depriving it of the presidency of the Boeotian League.

An attempt by the Spartans to expel it from the Delphic amphictyony is only frustrated by the intercession of Athens.

Cimon helps Aristides in 478 BCE to secure the transference from Sparta to Athens of the leadership of the recently liberated Greek maritime states.

Under the leadership of Themistocles and Cimon, the Athenians in the summer of 478 BCE reform the alliance into a new body, the Delian League (a modern expression).

Organized to preserve the freedom of the newly liberated Greek states and drive the Persians from the rest of the Ionian cities and islands in Asia Minor; the League also contemplates carrying the war into Persian territories.

Athens, by using monetary contributions from other member states to build its own military forces, will essentially transform the Hellenic League into its own empire.

Themistocles outwits the Spartans when they attempt to prevent Athens from rebuilding its defensive walls, but he fails to induce the people either to transfer their capital to Piraeus or, at this time, to reduce the powers of the Areopagus.

The Athenian people, after their tremendous war effort, are in a mood of reaction.

Themistocles, a strong democrat, is hated by the Athenian upper classes, who come to suspect him of plotting with the Persians, and, though praised (not by name) in Aeschylus' Persians (472 BCE), he is at last ostracized.

The eastern Greek allies revolt from Spartan control toward the end of 478 and at Delos offer their allegiance, through Aristides, to Athens.

The new alliance, established on the basis of equality of its members, convenes on the island of Delos, sacred to Apollo.

The original organization of the league, as sketched by Thucydides, indicates that all Greeks were invited to join to protect themselves from Achaemenian Persia.

Athens is in fact interested in further supporting the Ionians in Anatolia and exacting retribution from the Persians, whereas Sparta is reluctant to commit itself heavily overseas.

The Athenians are to supply the commanders in chief and to decide which states are to provide ships or money; money is to be received and controlled by ten Athenian treasurers (hellenotamiai).

Representatives of all member states, each with equal vote, are to meet annually at Delos, where the league's treasury is kept in the temple of Apollo.

The original membership probably includes most of the Aegean islands, except Aegina, Melos, and Thera, most of the cities of Chalcidice, the shores of the Hellespont and Bosporus, some of Aeolia, most of Ionia, and a few eastern Dorian and non-Greek Carian cities.

Apart from the big Ionian islands and some mainlanders, there are in fact Dorian members like Rhodes and Aeolians like Lesbos; there even are some non-Greeks on Cyprus, always a place with a large Semitic component. (Some Cypriot communities probably join at the outset.)

Some Thracian cities are surely enrolled very early.

Aristides, who is with Cimon one of the chief architects of the League, is chosen to assess the contributions to be paid by the member state; some contribute ships and men instead of money.

The Athenian fleet harasses Andros for supplying ships to Xerxes in 480.

The westernmost of the Cyclades, Kéa, inhabited since early Helladic times, had fought on the Greek side in the naval battles of Artemisium and Salamis (both 480 BCE) during the Greco-Persian Wars, and it subsequently joins the Delian League and the Athenian alliance.