In the meantime the Lacedaemonians prepared for their invasion of Attica, in accordance with their own previous resolve, and at the instigation of the Syracusans and Corinthians, who wished for an invasion to arrest the reinforcements which they heard that Athens was about to send to Sicily. Alcibiades also urgently advised the fortification of Decelea, and a vigorous prosecution of the war. But the Lacedaemonians derived most encouragement from the belief that Athens, with two wars on her hands, against themselves and against the Siceliots, would be more easy to subdue, and from the conviction that she had been the first to infringe the truce. In the former war, they considered, the offence had been more on their own side, both on account of the entrance of the Thebans into Plataea in time of peace, and also of their own refusal to listen to the Athenian offer of arbitration, in spite of the clause in the former treaty that where arbitration should be offered there should be no appeal to arms. For this reason they thought that they deserved their misfortunes, and took to heart seriously the disaster at Pylos and whatever else had befallen them. But when, besides the ravages from Pylos, which went on without any intermission, the thirty Athenian ships came out from Argos and wasted part of Epidaurus, Prasiae, and other places; when upon every dispute that arose as to the interpretation of any doubtful point in the treaty, their own offers of arbitration were always rejected by the Athenians, the Lacedaemonians at length decided that Athens had now committed the very same offence as they had before done, and had become the guilty party; and they began to be full of ardour for the war. They spent this winter in sending round to their allies for iron, and in getting ready the other implements for building their fort; and meanwhile began raising at home, and also by forced requisitions in the rest of Peloponnese, a force to be sent out in the merchantmen to their allies in Sicily. Winter thus ended, and with it the eighteenth year of this war of which Thucydides is the historian.
In the first days of the spring following, at an earlier period than usual, the Lacedaemonians and their allies invaded Attica, under the command of Agis, son of Archidamus, king of the Lacedaemonians. They began by devastating the parts bordering upon the plain, and next proceeded to fortify Decelea, dividing the work among the different cities. Decelea is about thirteen or fourteen miles from the city of Athens, and the same distance or not much further from Boeotia; and the fort was meant to annoy the plain and the richest parts of the country, being in sight of Athens. While the Peloponnesians and their allies in Attica were engaged in the work of fortification, their countrymen at home sent off, at about the same time, the heavy infantry in the merchant vessels to Sicily; the Lacedaemonians furnishing a picked force of Helots and Neodamodes (or freedmen), six hundred heavy infantry in all, under the command of Eccritus, a Spartan; and the Boeotians three hundred heavy infantry, commanded by two Thebans, Xenon and Nicon, and by Hegesander, a Thespian.
These were among the first to put out into the open sea, starting from Taenarus in Laconia. Not long after their departure the Corinthians sent off a force of five hundred heavy infantry, consisting partly of men from Corinth itself, and partly of Arcadian mercenaries, placed under the command of Alexarchus, a Corinthian. The Sicyonians also sent off two hundred heavy infantry at same time as the Corinthians, under the command of Sargeus, a Sicyonian. Meantime the five-and-twenty vessels manned by Corinth during the winter lay confronting the twenty Athenian ships at Naupactus until the heavy infantry in the merchantmen were fairly on their way from Peloponnese;thus fulfilling the object for which they had been manned originally, which was to divert the attention of the Athenians from the merchantmen to the galleys.
During this time the Athenians were not idle. Simultaneously with the fortification of Decelea, at the very beginning of spring, they sent thirty ships round Peloponnese, under Charicles, son of Apollodorus, with instructions to call at Argos and demand a force of their heavy infantry for the fleet, agreeably to the alliance. At the same time they dispatched Demosthenes to Sicily, as they had intended, with sixty Athenian and five Chian vessels, twelve hundred Athenian heavy infantry from the muster-roll, and as many of the islanders as could be raised in the different quarters, drawing upon the other subject allies for whatever they could supply that would be of use for the war. Demosthenes was instructed first to sail round with Charicles and to operate with him upon the coasts of Laconia, and accordingly sailed to Aegina and there waited for the remainder of his armament, and for Charicles to fetch the Argive troops.