Thucydides:
The History of the Peloponnesian War

Thucydides wrote the story of the first democracy in history, and of the fortunes and fall of its empire, but his pages contain the modern world-scene in miniature. The tale is told by a great political thinker, whose penetrating insight and dramatic power caused Macaulay to call him the 'greatest historian that ever lived.' His work, slightly abridged, is here presented in translation with an introduction and notes. [via]