Thucydides.

Written in the 5th Century BC, the events of the war between the Peloponnesian and Delian leagues are detailed by Thucydides. The leagues were led by Sparta and Athens respectively. The war marks a turning point in Ancient Greek history as the golden age of Greek dominance is changed in the region. What Thucydides achieves is one of the first real historical accounts detailing minute aspects of the war and how humanity records events in the centuries to follow.