I re-read this history (which is nearly two and a half thousand years old) recently, and it’s still amongst my favourites. The edition I have is published by Penguin, with translation by Rex Warner and introduction/notes by MI Finley.
Thucydides wrote an account of most of the Peloponnesian War, which occurred in the 5th century BC between Athens and Sparta, and their respective allies. It lasted decades and was rather complicated. Unlike some other ancient conflicts there weren’t persistent strong characters who defined the war (the exception might be Alcibiades, but only to an extent) which may be why it’s not quite as well-known as the Second Punic War or Alexander’s campaigns.
The author himself was an Athenian who played a brief role in the War before being exiled. He then spent years writing of the conflict, but appears to have died before he could finish it.
Thucydides wrote in a precise, factual manner. Although some elements are guessed at (the specific wording of speeches, for example) most of these are guided by speaking to witnesses or documentation. Although sometimes coloured by personal views (he was not a fan of Cleon), he does not appear biased in general terms for or against Athens, Sparta, Syracuse or any other player in the game, and is not afraid to condemn his own side when he felt they were in the wrong.
His approach (contrary to many ancient historians) of including specific numbers where possible and giving detail as to battle and siege where it existed enables a more lively and accurate account to come out. Whilst written with a cool, calculated hand, Thucydides does a great job of portraying success and plight as the fortunes of war ebbed and flowed.
As he himself wrote, this is intended to be an objective account of what happened that will stand the test of time, and on that score it’s a clear success.
The sometimes dry style and willingness of the writer to use an eight clause sentence if that’s what it takes to write what he wants to write may mean this isn’t ideal for a beginner to classical history (that said, I got it fairly early on and didn’t have particular problems). The footnotes and appendices do a good job of explaining what needs to be explained. There are also several maps in the back (perhaps a few more would’ve been helpful, though maybe I’m being picky).
The only real downside is that the book is unfinished, but it is still substantial, covering over two decades and 600 pages.
I’d advocate reading this after Thucydides, but can also strongly recommend the excellent single volume history of the Peloponnesian War by Donald Kagan.