If I understand correctly, Thucydides actually called his book just History. If that is correct, then at what point did it become known as History of the Peloponnesian War?
EDIT: Here is how he begins:
Thucydides, an Athenian, wrote the history of the war between the Peloponnesians and the Athenians, beginning at the moment that it broke out, and believing that it would be a great war, and more worthy of relation than any that had preceded it.
Come to think of, I ran a search on the Perseus website (it's great we have it!) and it turns out Thucydides did use the term once:
The Median war, the greatest achievement of past times, yet found a speedy decision in two actions by sea and two by land. The Peloponnesian war was prolonged to an immense length, and long as it was it was short without parallel for the misfortunes that it brought upon Hellas.
Still, I think it is not obvious that this term should have become the title of the whole book. Or am I missing something obvious here?