Thucydides, an Athenian, wrote the history of the war between the Peloponnesians and the Athenians, beginning at the moment that it broke out, and believing that it would be a great war and more worthy of relation than any that had preceded it. (1.1.1)[2]
A Theban force, a little over three hundred strong ... about the first watch of the night, made an armed entry into Plataea, a town of Boeotia allied with Athens. The gates were opened to them by a Plataean called Naucleides, who with his faction had invited them in, meaning to put to death the citizens of the opposing faction, bring over the city to Thebes, and thus obtain power for themselves. (2.2.1; modified)
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 … Indeed this was how they got in so easily without being observed, as no guard had been posted beforehand. (2.2.3)
having, as they considered, suffered evil at the hands of the Plataeans , they brought them in again, one by one and asked each of them the same question … whether they had done the Lacedaemonians and their allies any service in the war; … and upon their saying that they had not took them out and slew them all without exception.” (3.68.5)
The adverse attitude of the Lacedaemonians in the whole Plataean affair was mainly adopted to please the Thebans, who were thought to be useful (ophelimous) in the war at that moment just starting. Such was the end of Plataea in the ninety-third year after they became the allies of the Athenians. (3.68.4; modified)
Vengeance is not necessarily successful because wrong has been done, or strength sure because it is confident; but the incalculable element (to astathmeton) in the future exercises the widest influence, and is the most treacherous…. (4.62.4; cf. Euripides Orestes 981)