Book III, chaps. liii-lix

Plataean speech to the Spartan judges

The Plataeans, as Daniel Holmes has pointed out, appeal to the
Spartan sense of honor and concern for their reputation.
Although the Spartans had promised to punish no one against
justice (52.2), they now appear to identify justice with their
own advantage. As the Plataeans admonish them, "For if you shall
decide the question of justice by such considerations as your
immediate advantage and their [the Thebans'] hostility, you will
show yourselves to be, not true judges of what is right, but
rather to be mere slaves of expediency."

The Plataeans also recount their own glorious history at the time
of the Persian Wars (which contrasts to the Thebans' "Medizing,"
i.e. siding with Persia) and their help to Sparta after the
earthquake and the revolt of the Helots. (This incident was the
cause of the first open break between Athens and Spartan
following the Persian Wars. I.ci-cii.) The Plataeans then imply
that their alliance with Athens occurred after these events
(lv.i), whereas it in fact preceded them (a fact of which the
Thebans will make use in their reply -- it enables the Thebans to
assert that Plataea's anti-Persian stance was just another sign
of its loyalty to Athens.)

The Plataeans even invoke the Spartans' "ideological"
justification for the war: "we adjure you ... not, while
liberating the rest of the Hellenes, to bring utter destruction
upon us." (lix.4)

This incident reminds us of the story of the Spartan admiral
Alcidas, who automatically put to death the prisoners he took in
Ionia (citizens of Chios and various other cities subject to
Athens), until some pro-Spartan Samians pointed out to him "that
he was not going the right way to free Hellas in massacring men
who had never raised a hance against him, and who were not
enemies of his, but allies of Athens against their will, and that
if he did not stop he would turn many more friends into enemies
than enemies into friends." (III.xxxii.2) Alcidas immediately
sees the strength of this point and changes his behavior:
Thucydides' point seems to be that, prior to the Samians'
remonstrance, a Spartan like Alcidas hadn't even considered
whether killing his prisoners was expedient or not.

The Spartan behavior here seems similar, but as the Plataeans
suspect (and Thucydides later confirms), the Spartans actually
have a reason for killing the Plataeans -- to please the Thebans,
whom the Spartans saw as important allies in the war. (lviii.4)

In any case, this story, coming right after the Mytilenean debate
at Athens, shows the Spartans in a particularly unpleasant light.
They promise to deal with the Plataeans according to justice
(Paches made no such promise to the Mytileneans III.xxviii -- all
the Mytileneans are promised is the opportunity to plead their
case before the Athenian assembly), but it is evident that they
have no such intention. Their initial question to the capture
Plataeans explicitly equates justice with support for Sparta in
the war, a totally "ideological" approach.

By comparison with the Athenians, what is most striking is the
absence of any Spartan deliberation about the justice or
expediency of putting the Plataeans to death -- this would appear
to be a matter of course for them. (This recalls Cleon's claim
that sparing the Mytileneans is such a preposterous position that
one could take it only if one had been bribed or if one wanted to
"show off" by cleverly maintaining an absurdity. III.xxxviii.2-
7 Presumably, Cleon would have been willing to have dispensed
with the debate altogether had that been possible in Athens.)