So we discussed Thucydides today. We heard the argument that the Spartans were more honorable, and some apparently believe that is why they won. After all, if you've got to side with a major power, it might as well be one that keeps its word. Thus, it could be argued that Thucydides's writings contain a lesson in morality for great powers. With all due respect, I must disagree with this interpretation. History is never this clear-cut.
Another interpretation of Thucydides is a comparison of the Athenian and Spartan governments. Athens was a prosperous democracy; Sparta was an militaristic aristocracy. In Athens, ideas were freely discussed, and any citizen (citizenship being far more narrowly defined then) could hold office. Spartan society virtually centered on maintaining control of the helots, the slave caste. I would think that parallels between the US and Athens would be obvious, while parallels between Sparta and various authoritarian governments would be equally plain. Athens lost. I have no doubt that militaristic societies such as Nazi Germany used Thucydides to support their wars. "Athens lost. The US will, too. Why? Because democracies are weak." I don't agree with this interpretation. I'm just using it as an example of a potential lesson in Thucydides.
There are a lot of lessons in Thucydides; many of them are still relevant. Thucydides is like virtually any historian; what get out of his writings depends on how you approach them.