Thucydides served as elected general (strategos) for Athens, and it is likely that he had (perhaps extensive) personal experience of warfare. His work is therefore an important guide both to the practicalities of warfare in 5th-century BCE Greece and to the wider function(s) that war played in politics and society. This chapter analyses what the History tells us about the ‘art of war’ in this period, discussing the use of land troops (light-armed soldiers and cavalry as well as hoplites) and naval forces. It discusses military strategy and tactics, the nature of combat and the consequences of warfare, for non-combatants as well as soldiers.