The basic argument here is that Thukydides may well be decently reliable in his reportage, but that his interpretation of the events reported is subject to challenge on numerous counts, such as the causes of the Peloponnesion War, the effectiveness of Pericles, the meaning of Athenian democracy, the scope of the conflict, and the responsibility for the Sicilian disaster (i.e., Kagan makes a decent case that Thukydides' favorite, Nicias, should be cast in judgment).
The fundamental tool of analysis is that Thukydides is a revisionist, even though he is the first true historian in the modern sense (Herodotos doesn't count for Kagan), which nicely renders all history as revisionism. The target of revision was, for Kagan, the conventional opinions of Athenians at the time of Thukydides' writing; Kagan does not fail to point out that Thukydides is himself not exactly sympathetic to Athens after his exile during the war.
The text is short and sweet, and though I may prefer de Ste. Croix's reading of Thukydides, this is certainly worthwhile for ancient history nerds, art of war geeks, Alcibiades epigones, and Peisistratid apologists.