The allies of the two belligerents were the following. These were the allies of Lacedaemon: all the Peloponnesians within the Isthmus except the Argives and Achaeans, who were neutral; Pellene being the only Achaean city that first joined in the war, though her example was afterwards followed by the rest. Outside Peloponnese the Megarians, Locrians, Boeotians, Phocians, Ambraciots, Leucadians, and Anactorians. Of these, ships were furnished by the Corinthians, Megarians, Sicyonians, Pellenians, Eleans, Ambraciots, and Leucadians;and cavalry by the Boeotians, Phocians, and Locrians. The other states sent infantry. This was the Lacedaemonian confederacy. That of Athens comprised the Chians, Lesbians, Plataeans, the Messenians in Naupactus, most of the Acarnanians, the Corcyraeans, Zacynthians, and some tributary cities in the following countries, viz., Caria upon the sea with her Dorian neighbours, Ionia, the Hellespont, the Thracian towns, the islands lying between Peloponnese and Crete towards the east, and all the Cyclades except Melos and Thera. Of these, ships were furnished by Chios, Lesbos, and Corcyra, infantry and money by the rest. Such were the allies of either party and their resources for the war.
Immediately after the affair at Plataea, Lacedaemon sent round orders to the cities in Peloponnese and the rest of her confederacy to prepare troops and the provisions requisite for a foreign campaign, in order to invade Attica. The several states were ready at the time appointed and assembled at the Isthmus: the contingent of each city being two-thirds of its whole force. After the whole army had mustered, the Lacedaemonian king, Archidamus, the leader of the expedition, called together the generals of all the states and the principal persons and officers, and exhorted them as follows:
"Peloponnesians and allies, our fathers made many campaigns both within and without Peloponnese, and the elder men among us here are not without experience in war. Yet we have never set out with a larger force than the present; and if our numbers and efficiency are remarkable, so also is the power of the state against which we march. We ought not then to show ourselves inferior to our ancestors, or unequal to our own reputation. For the hopes and attention of all Hellas are bent upon the present effort, and its sympathy is with the enemy of the hated Athens. Therefore, numerous as the invading army may appear to be, and certain as some may think it that our adversary will not meet us in the field, this is no sort of justification for the least negligence upon the march; but the officers and men of each particular city should always be prepared for the advent of danger in their own quarters. The course of war cannot be foreseen, and its attacks are generally dictated by the impulse of the moment; and where overweening self-confidence has despised preparation, a wise apprehension often been able to make head against superior numbers. Not that confidence is out of place in an army of invasion, but in an enemy's country it should also be accompanied by the precautions of apprehension: troops will by this combination be best inspired for dealing a blow, and best secured against receiving one. In the present instance, the city against which we are going, far from being so impotent for defence, is on the contrary most excellently equipped at all points; so that we have every reason to expect that they will take the field against us, and that if they have not set out already before we are there, they will certainly do so when they see us in their territory wasting and destroying their property. For men are always exasperated at suffering injuries to which they are not accustomed, and on seeing them inflicted before their very eyes; and where least inclined for reflection, rush with the greatest heat to action. The Athenians are the very people of all others to do this, as they aspire to rule the rest of the world, and are more in the habit of invading and ravaging their neighbours' territory, than of seeing their own treated in the like fashion. Considering, therefore, the power of the state against which we are marching, and the greatness of the reputation which, according to the event, we shall win or lose for our ancestors and ourselves, remember as you follow where you may be led to regard discipline and vigilance as of the first importance, and to obey with alacrity the orders transmitted to you; as nothing contributes so much to the credit and safety of an army as the union of large bodies by a single discipline."With this brief speech dismissing the assembly, Archidamus first sent off Melesippus, son of Diacritus, a Spartan, to Athens, in case she should be more inclined to submit on seeing the Peloponnesians actually on the march. But the Athenians did not admit into the city or to their assembly, Pericles having already carried a motion against admitting either herald or embassy from the Lacedaemonians after they had once marched out.
The herald was accordingly sent away without an audience, and ordered to be beyond the frontier that same day; in future, if those who sent him had a proposition to make, they must retire to their own territory before they dispatched embassies to Athens. An escort was sent with Melesippus to prevent his holding communication with any one. When he reached the frontier and was just going to be dismissed, he departed with these words: "This day will be the beginning of great misfortunes to the Hellenes." As soon as he arrived at the camp, and Archidamus learnt that the Athenians had still no thoughts of submitting, he at length began his march, and advanced with his army into their territory. Meanwhile the Boeotians, sending their contingent and cavalry to join the Peloponnesian expedition, went to Plataea with the remainder and laid waste the country.