While Herodotus is considered 'the father of history,' Thucydides is perhaps the first true historian. Unlike Herodotus, Thucydides writes his History of the Peloponnesian War Between Athens and Sparta with the goal of writing a truly accurate history.
Thucydides was able to achieve this accuracy because he himself lived through, and briefly even participated in, the Peloponnesian Wars. Born around 460 BC, Thucydides caught the plague early in the Peloponnesian War, and later served as a general. When he failed to protect the Athenian colony of Amphipolis from the Spartans, Thucydides was condemned and exiled in absentia. He would only return to Athens twenty years later.
Thucydides realized the importance of the events that he was living through, and set about accurately recording them for posterity. Exiled from Athens, Thucydides was in a unique position to observe both sides with relative neutrality. Beyond relating an accurate account of the war, he also attempted to scientifically analyze the reasons behind the war, dismissing gods and religions, which are so prevalent in Herodotus' History.
In order to analyze the positions of the warring sides, Thucydides incorporates several speeches by generals and politicians. While he himself admits that they might not have the same degree of accuracy as the rest of the work, they provide the reader valuable insight into the positions of various men and states.
No doubt influenced by the new scientific writers of his time, such as Hippocrates, Thucydides strived for, and achieved, a degree of accuracy unmatched by any other ancient author.
Thucydides. The Peloponnesian Wars.
Oxford Concise Companion to Classical Literature. ed. Howatson and Chilvers. Oxford. New York, 1993.