There are two basic things to talk about when it comes to Thucydides great work 'The Peloponnesian War'. The first to talk about is what his writing approach meant to his contemporaries and to posterity. On the eve of the war, Thucydides had a feeling that this would be an important conflict and he wanted to make a good record. What followed is sometimes called scientific history, but I think a better term would be 'journalistic history'. That is, he tried to make a true record of what happened. He stayed as close to the facts as he possibly could. He also spoke with people from both sides of the war. The result is fairly balanced. Or at least, both Athens and Sparta spend time as heroes and villains.
You can read echoes of Thucydides in more modern efforts at history. My all time favorite history book is Garrett Mattingly's 'The Armada'. He details the attack of the Spanish Armada. Mattingly talks about the political situation in each of the principal countries before the war. He talks about the main players. And he spends time discussing popular conceptions of what happened. Sound familiar? It's the same template that Thucydides created.
And the second thing? I think that will have to wait for another post.
I do want to say that Thucydides isn't an easy read. It helped me tremendously to read from the Landmark version. I mentioned it here and I want to recommend it again. It was especially helpful to have maps of the many places that are talked about. It was also helpful to have so much information cross referenced.