Of particular interest during this time is the section concerning the Athenian invasion of Sicily. Thucydides describes the debate in Athens in some detail and then, also, the invasion. The debate included such topics as the defense of democracy and the rights of smaller nations to self-dermination. I have linked an online version of the book and a commentary. However, if you don't already have a copy from High School or College, they are available in Paperback form at your local Book-Mart.