Found here——Kraut, Richard.  The Cambridge Companion to Plato. New York, NY:  Cambridge University Press, 1992.

“The Peloponennisian War created the sorts of tension in Athens that would appear to support Thucydides’ analysis.  Obligations to the community required greater sacrifice and presented a clearer conflict with the self-seeking “Homeric” pursuit of one’s status, power and pleasure.  In political terms, people had to decide whether or not to plot against the democracy to bring off an Olgarchic coup.  In moral terms they had to decide whether or not to ignore the demands of the community, summed up in the requirements of “justice,” in favor of their own honor, status, power, and in general their perceived interest.  Plato was familiar with people who preferred self-interest over other-regarding obligation; his own relatives, Critias and Charmides, made these choices when they joined the Thirty Tyrants.

Arguments from natural philosophy did not restrain people like Critias and Charmides.  Democritus argues unconvincingly that the requirements of justice and the demands of nature, as understood by Atomism, can be expected to coincide. Protogoras rejects the view that moral beliefs are true and well grounded only if they correspond to some reality independent of believers; admittedly they are matters of convention, but so are all other beliefs about the world.  This line or argument removes any ground for preferring nature over convention, but at the same time seems to remove any rational ground for preferring one convention over another.”