Thucydides was born in around 460 BC in Alimos, Athens. Little is known about his early life, and all that scholars know of him at all is taken from what little he wrote about himself in his works. We know that he would have been around 30 when the Peloponnesian War started. He was an ex-general at that time, who had been exiled and removed from his post after failing to defend the city of Amphipolis during a conflict with Sparta in 424.
Thucydides is best known as the author of the History of the Peleponnesian War, which documents the battles of the 5th century between the two Greek poleis Athens and Sparta. His History is considered the first account that discusses a conflict from both a political and moral perspective. Thucydides died suddenly in Athens in around 400 BC, after the Peleponnesian War had ended but before he was able to complete his account of it. The last part of his History was finished by the philosopher Xenophon.