A theme that comes out in book VIII is the importance of Persia. Suddenly, somehow, the action shifts to the Athenian colonies on what-is-now-Turkey and the offshore islands - Chios, Lesbos, Samos, Rhodes - and the way the Persian power can become the decisive force if it is thrown behind one side or the other; and the scramble for support. The apparent pivotal position of Alcibiades is odd too. As the commentary says, it looks as though T only realised this stuff late on: so that it makes its way into book VIII, but not into re-writes of the earlier books.
I found the description of the Spartan disaster at Sphacteria particularly interesting. The Spartans had a fearsome reputation, and even at more-than-4-to-1 odds the Athenians hesitated to attack the isolated troops on the island. But once they did and it worked, suddenly they were no longer afraid. Or, consider the way Corinth got itself tied into knots over Epidamnus in the lead-in to the war.
So there are any number of mottoes you can take from the book. Above all, there's a what-if (which of course the book doesn't address; its a history): the war killed lots of people, wasted huge resources, and weakened Greece, leaving it prey to Philip and later the Romans. Could they have done better? The book makes it moderately clear, or at least plausible, that the Spartans were actually thinking; and the Athenians too, at least sometimes and when well lead. So it might have been possible - perhaps if they'd had some examples before them, which of course they hadn't - to realise that war would be dreadful and was happening because of this clash in the face of expanding Athenian power, and they desperately needed to come to an accommodation to avoid that.
How does that affect how I think about the Ukraine?
Um, I seem to have drifted away from reviewing the book. But that's great: you see, its a book that provokes thought. But maybe only if you have context, so I wouldn't recommend it unless you do want to try and think like this.