The Peloponnesian War happened the best part of 25 centuries ago between the Ancient Greek city states of Sparta and Athens. Athens was a sea power that, perhaps counter-intuitively for a democratic state, maintained an empire and forced lesser powers to pay tribute. Sparta enjoyed dominion over the Peloponnesus, the peninsula of Greece, through a mighty reputation for prowess in battle.
Thucydides was a brilliant writer, possibly the earliest proper historian. Yes, Herodotus was first, but Herodotus also talked nonsense at times. Thucydides related events which he not merely heard of second hand, but participated in, to a limited extent.
He was a minor and not terribly successful Athenian general, but did spend many years working on his excellent book. Like Polybius, he is more concerned with truth and fact than Livy-like exaggerations, although it is probable that in parts his mind was swayed by personal interest.
One similarity with Livy is that Thucydides does sometimes invent speeches when he cannot know what was said. Whilst not strictly historical, I find that (for both authors) this does actually add to the value of the history, and the sentiments expressed by Thucydides are likely to at least be approximately accurate.
The war lasted for decades and the two principle cities involved were supported by scores of weaker allies who were either strong-armed or persuaded through friendship or reason to side with Sparta or Athens. Unlike the Second Punic War, which was dominated by the excellent Hannibal, there was no one predominant figure, probably due to the lengthy nature of the war.
Thucydides writes in a detailed, dedicated way, and is quite happy to have a sentence that lasts 8 lines and has 12 clauses if it makes his point. He does an excellent job of describing not only what happened, but explaining why it happened. In addition to detail he also paints a good picture of the more general situation and atmosphere in a given time and place. For example, at the height of the conflict numerous cities were torn between siding with Athens or Sparta and the two sides within a city would embark upon bloody rampages, moderation was accorded cowardice and murder was rife.
I would also say that the book is as relevant today as it was the day it was written. I imagine that if Blair and Bush had read it, and viewed with horror the unnecessary expedition the Athenians mounted to try and defeat Syracuse when Sparta was on her knees they may have been dissuaded from invading Iraq when Afghanistan seemed to be going so well.
The book is not especially easy to read, but it is absolutely worth the effort. It chronicles the ebb and flow of an ancient conflict but the principles involved are as true today as they ever were.
I can also recommend Donald Kagan’s excellent book (The Peloponnesian War: Athens and Sparta in Savage Conflict) which is festooned with fantastic maps and puts a lot of what Thucydides writes into a more understandable context.