The strategic importance of Megara is immediately demonstrated by the appearance, for the first time in twelve years, of a Spartan army north of the Isthmus in Attica, under King Pleistoanax, who reaches Eleusis and the Thriasian plain.

Pericles thinks and acts swiftly.

The details will never be fully known, but, possibly by bribery and certainly by negotiation, it is arranged that Athens, in return for Spartan noninterference in Euboea or invasion of Attica, will give up its mainland possessions (Boeotia, the Megarid, and certain Peloponnesian sites) and confine itself to a largely maritime empire.

The Spartan army retires, Euboea is quickly reduced, and the arrangement is ratified by the Thirty Years' Peace (winter 446-445), possibly negotiated by Callias.

For Athens, the essential loss is that of Megara, which means that a Spartan army can appear in Attica at any time.

Percicles' immediate construction of a third Long Wall to improve the defenses of Athens and the port of Piraeus shows that Pericles doubts the stability of the settlement with Sparta and sees the need to develop an alternative basic strategy for Athens.

Henceforth, in effect, Athens can be turned into an island at will.