Thucydides was an Athenian general and historian who lived in the fifth century BCE and wrote The History of the Peloponnesian War, the account of the twenty-seven-year conflict between Athens and Sparta that ended Athenian hegemony in the Greek world. He did not finish it. The narrative breaks off in 411 BCE, seven years before the war ended. What he left behind is nonetheless the foundational text of realist political thought and the first serious attempt to explain why states go to war.