Thus the provision of forces by each state to act against Persia was changed in effect into a tribute for the expansion of the Athenian fleet.  The continuous development of the power of Athens had been checked only momentarily by her disastrous Egyptian expedition.  Her nominal allies found themselves actually her tributary dependencies, and various attempts to break free from her yoke had made it only more secure and more burdensome.

Hence the warlike decision of Sparta was welcomed by others besides Corinth.  But diplomatic demands preceded hostilities.  Sparta and Athens sent to each other summons and counter-summons for the “expulsion of the curse,” that is of all persons connected with certain families which lay under the curse of the gods.

In the case of Athens, this amounted to requiring the banishment of her greatest citizen and statesman, Pericles.  To this the Spartans added the demand that the Athenians should “restore the freedom of Hellas,” and should specifically remove certain trading disabilities imposed on the people of Megara.

At this crisis Pericles laid down the rules of policy on which Athens ought to act—­rules which required her to decline absolutely to submit to any form of dictation from Sparta.  When a principle was at stake, it made no difference whether the occasion was trivial or serious.  Athens could face war with confidence.  Her available wealth was far greater—­a matter of vital importance in a prolonged struggle.  Her counsels were not divided by the conflicting interests of allies all claiming to direct military movements and policy.  Her fleet gave her command of the sea, and enabled her to strike when and where she chose.  If Peloponnesian invaders ravaged Attica, still no permanent injury would be done comparable to that which the Athenians could inflict upon them.  The one necessity was to concentrate on the war, and attempt no extension of dominion while it was in progress.

War was not yet formally declared when the Thebans attempted to seize Plataea, a town of Boeotia, which had long been closely allied to Athens.  The attempt failed, and the Thebans were put to death; but the Plataeans appealed to Athens for protection against their powerful neighbour, and when the Athenian garrison was sent to them, this was treated as a casus belli.

Preparations were urged on both sides; Sparta summoned her allies to muster their contingents on the Isthmus for the invasion of Attica, nearly all the mainland states joining the Peloponnesian league.  The islanders and the cities in Asia Minor, on the other hand, were nearly all either actually subject to Athens or in alliance with her.

As Pericles advised, the Athenians left the country open to the ravages of the invading forces, and themselves retired within the city.  In spite of the resentment of those who saw their property being laid waste, Pericles maintained his ascendency, and persuaded the people to devote their energies to sending out an irresistible fleet, and to establishing a great reserve both of ships and treasure, which were to be an annual charge and brought into active use only in the case of dire emergency.  The fleet sailed round the Peloponnese, and the ravages it was able to inflict, with the alarm it created, caused the withdrawal of the forces in Attica.