Well, yes.

Thucydides didn't complete the history of the Peloponnesian war, and the conclusion is usual based on Xenophon (who is easily one of the greatest writers of all time). Kagan integrated the two, added in a couple more primary and ancient secondary sources, deconflicted a bunch of stuff, and put it into clear prose.

Thucydides is good, and stands alone pretty well, but Kagan gives you a more readable and complete picture.