When I was enrolled in Naval War College, Thucydides' "History of the Peloponnesian Wars" was a major work to be read and discussed for ramifications on modern warfare. Especially the forming, maintenance and strategic value of coalitions, and shifts in the character and goals of the participants....that occur over a protracted conflict (not necessarily 30 years). Some works, and the lessons they can convey about historical "lessons learned", are timeless.
I think the Peloponnesian Wars really are a lesson for our times. For example, the Spartan war effort that finally beat Athens was largely funded by Persia. I think the war was largely a draw until then.
For all that is written about the suffering of the Athenians, Sparta suffered badly as well. The armies were actually well matched, as I recall, but the forces of Athens were spread all over the Agean, the Eastern Med. from Sicily to Cyprus, and the Black Sea. A scattering of forces that was to prove fatal.
Or at least, that's my amateur, studied only for fun, take on the matter.