Thucydides opens his History by introducing himself and explaining how his methods and intentions differ from those of poets. His evidence is more reliable because it’s based on cross-referenced eyewitness accounts. He includes speeches heard firsthand either by himself or his informants. His account is more trustworthy because poets are known to exaggerate. Thucydides claims that he’s not trying to entertain or compete for prizes, as poets do at festivals, but to inform his audience. While his account may be less enjoyable, it may be more useful, if it helps those who wish to understand past events that will probably recur in the future since human nature is eternally unchanging.

Speaking of the events that led to the war between Athens and Sparta, Thucydides explains that Greek city-states collaborated successfully to repel the Persian invasion, but immediately following that success, they split into two factions. Democratic Athens led one alliance while oligarchic Sparta headed the other. Athens was the undisputed sea power while Sparta’s army dominated on land. Athens’s allies were obliged to pay tribute in the forms of money or navies while Sparta’s were not, though Sparta promoted oligarchic rule in their allied states and received the promise of military aid from them.