There's a bit more to it than that. Athenian historian
Thucydides actually wrote a long-ass book on the subject. Himself a
general in that
conflict, he wrote much of his
history from first-hand
experience. The rest he either made
educated guesses about or sought
reliable witnesses. Failing that, he gathered what he could from drunken
sailors at some waterfront bar he used to hang out in. (Perhaps that's not entirely
true, but I'm using the Thucydidean method here.)
When you say "long war," you're quite right. We're talking decades here. More than just Athens and Sparta were involved...they each had bunches of allies (a classical euphemism for "subjects") who were coerced into the fight. There was a whole powder-keg situation in the Hellenic world at the time...alliances and cross-deals...ranging from the Greek mainland to Asia Minor, all throughout the Mediterranean, and back to Sicily and southern Italy. Most of these regions saw action, and many cities of 30,000 or more inhabitants were completely reduced.
A nasty business, but it makes for really good reading. I heartily recommend that anyone with an even vague interest in classical history check out Thucydides' History of the Peloponnesian War.