I’m a fan of Donald Kagan’s four-volume study (1, 2, 3, 4) of the Peloponnesian War, which includes some important information you won’t get from Thucydides and Xenophon, as well as a relief from their anti-democratic bias. Anyone with a interest in Greek history will read it with profit.
Unfortunately, along with Kagan’s appreciation for Athens’ democratic institutions (for my own defense of which see here and here) comes a tendency to gloss over or justify Athens’ imperialist foreign policy. I haven’t read Kagan’s condensed one-volume version, but this review of it strikes me as a fair assessment of that aspect of the longer version too. Kagan is right, of course, that Sparta was not the innocent victim that Thucydides sometimes suggests. But Kagan leans too far in the other direction. (It’s no coincidence that Kagan is also one of the signatories of this neocon screed.)
One thing I think Hans Hoppe is right about is that domestically liberal societies often tend to have aggressive foreign policies simply because economic freedom makes them wealthy enough to afford such policies. (I actually said this before I read Hoppe, here for example.) Athens seems like a good example of this phenomenon.