In documenting the Peloponnesian war, the defeat of the democracy of Athens by the oligarchy of Sparta in the 5th century BC, Thucydides invented a completely new approach to reporting events. Assigning nothing to the will of the gods, his history was instead impartial, evidence-based and informed by his understanding of human nature. His work has shaped attitudes towards military intervention for 2,400 years and continues to offer profound insights into the challenges facing humanity today.