Thucydides (born c. 460–455 BCE) wrote a valuable first-hand account of the Peloponnesian War (History of the Peloponnesian Wa) and improved the way in which history was written.
Thucydides wrote his history based on information about the war from his days as an Athenian commander and interviews with people on both sides of the war. Unlike his predecessor, Herodotus, he didn't delve into the background but laid out the facts as he saw them, chronologically. We recognize more of what we consider the historical method in Thucydides than we do in his predecessor, Herodotus.