The History of the Peloponnesian War

An account of the Peloponnesian War in Ancient Greece, fought between the Peloponnesian League (led by Sparta) and the Delian League (led by Athens). It was written by Thucydides, an Athenian general who served in the war. It is widely considered a classic and regarded as one of the earliest scholarly works of history. The History was divided into eight books by editors of later antiquity. - Source

There's a text-only version available for download here: http://classics.mit.edu/Thucydides/pelopwar.mb.txt (translation by Richard Crawley).