The theme of the inevitability of a war between Athens and Sparta runs through Thucydidesbook I, which makes the reader focus not on 'responsibility' but on 'nature' and its irreversibledynamic. It not only shows that war between the Athenians and the Spartans was unavoidableand that no poleis or individuals should be held responsible for it, but also describes the stepsby which the two sides became gradually aware that war was unavoidable. Thucydides disclosesthe processes that made the imminence of war a certainty in the mind of both the Athenians andthe Spartans in 433-432 BC; such awareness worked on a psychological level, predeterminingwar before it broke out.