The Eight Books of Thucydides' History, translated by R. Crawley.
Chapter VI : Beginning of the Peloponnesian War—First Invasion of Attica—Funeral Oration of Pericles
Chapter VII : Second Year of the War—The Plague of Athens—Position and Policy of Pericles—Fall of Potidaea
Chapter VIII : Third Year of the War—Investment of Plataea—Naval Victories of Phormio—Thracian Irruption into Macedonia under Sitalces
Beginning of the Peloponnesian War—First Invasion of Attica—Funeral Oration of Pericles
The war between the Athenians and Peloponnesians and the allies on either side now really begins. For now all intercourse except through the medium of heralds ceased, and hostilities were commenced and prosecuted without intermission. The history follows the chronological order of events by summers and winters.
The thirty years' truce which was entered into after the conquest of Euboea lasted fourteen years. In the fifteenth, in the forty-eighth year of the priestess-ship of Chrysis at Argos, in the ephorate of Aenesias at Sparta, in the last month but two of the archonship of Pythodorus at Athens, and six months after the battle of Potidaea, just at the beginning of spring, a Theban force a little over three hundred strong, under the command of their Boeotarchs, Pythangelus, son of Phyleides, and Diemporus, son of Onetorides, about the first watch of the night, made an armed entry into Plataea, a town of Boeotia in alliance with Athens. The gates were opened to them by a Plataean called Naucleides, who, with his party, had invited them in, meaning to put to death the citizens of the opposite party, bring over the city to Thebes, and thus obtain power for themselves. This was arranged through Eurymachus, son of Leontiades, a person of great influence at Thebes. For Plataea had always been at variance with Thebes; and the latter, foreseeing that war was at hand, wished to surprise her old enemy in time of peace, before hostilities had actually broken out. Indeed this was how they got in so easily without being observed, as no guard had been posted. After the soldiers had grounded arms in the market-place, those who had invited them in wished them to set to work at once and go to their enemies' houses. This, however, the Thebans refused to do, but determined to make a conciliatory proclamation, and if possible to come to a friendly understanding with the citizens. Their herald accordingly invited any who wished to resume their old place in the confederacy of their countrymen to ground arms with them, for they thought that in this way the city would readily join them.
Reference address : https://www.ellopos.net/elpenor/greek-texts/ancient-greece/thucydides/history-2.asp