Thucydides was an ancient Greek historian, best known for his work "History of the Peloponnesian War." He lived during the 5th century BCE and provided a detailed account of the conflict between the city-states of Athens and Sparta. His writing is notable for its rigorous standards of evidence and analysis, making it one of the earliest examples of historical methodology. Thucydides was also a general in the Athenian army, which gave him firsthand experience of the war. His insights into power dynamics, human nature, and political ethics have influenced historians and political theorists throughout history, including figures like Machiavelli and Hobbes.