It made no "long speech no one would believe, of fine moral arguments" (Woodruff 103). However, Pericles' claim that Athens did not need support to uphold its borders falls completely flat in the face of Athenian desperation to hold onto its empire. Thucydides, writing with the benefit of hindsight and the knowledge of the outcome of the war shows the Athenians in a far more ambiguous light than Pericles the politician, who was speaking to memorialize the dead. Thucydides shows the cowardice of the Athenians during the plague, as Athenians even turned against one another, in fear -- another example of might making right. Thucydides somewhat disenchanted view of Athens is partially due to his status as an exile -- he was deprived of his command due to no real fault of his own (he was unable to reach a critical strategic location in time to prevent it falling to the Spartans). Thus Thucydides was able to travel around the region and see the war from both sides: "I was able to observe things more closely and without distraction" (Woodruff 102).

While Athens argued that its empire had been acquired freely (in a kind of manifest destiny of democracy that echoes past justifications of U.S. imperialism) and thus morally, Athens' actions in defense of that empire proved otherwise. According to Thucydides, the Peloponnesian War was costly both in terms of capital and human resources, and it also propelled the 'worst' of Athenian society to the forefront of the political leadership (most notably the populist military leader Cleon) while depriving the city of its best and brightest. Athens became morally corrupt in its actions abroad -- and at home as a result of the war, as during war 'might makes right' by virtue of necessity, even for the most moral of nations.
Work Cited

Thucydides. On justice, power, and human nature: the essence of Thucydides' History of the Peloponnesian War. Paul Woodruff (Ed.) Hackett, 1993.