Sieges—whether Sparta’s successful attack on Plataea or Athens’ ruination of Melos—were often not explicable in a traditional strategic calculus of cost versus benefits. After all, what did the possession of Plataea do for the Spartan cause? How was Athens made more secure, wealthier, or stronger by taking Melos? The rent from the farms of the Athenian colonists who settled in the surrounding countryside after the city fell could hardly have paid the cost of the long siege. Nor would the sale of captives into slavery recover the expenses of the besiegers. Instead, the efforts to storm recalcitrant cities seemed to confer enormous psychological implications on the reputation and competence of the two powers. Letting Plataea defiantly stand apart from Thebes or Mytilene boast of its independence was seen as a contagion that could weaken the entire system of alliances that had grown up after the Persian Wars.
— Victor Davis Hanson, A War Like No Other, p. 178