Another on-line source defines anabasis as "1. a military advance 2. a difficult and dangerous military retreat". Now this would seem an unlikely pair of contrary meanings, creating a potential for a very serious mistake.

But from what I've been able to find, anabasis has only the former meaning; the term for the latter is catabasis or katabasis. (Compare anabatic/katabatic and anaphora/cataphora in the Wordcraft Dictionary.)

Did the other source simply err on anabasis, or is there support for that second meaning?

The "military retreat" meaning of anabasis seems to stem from Xenophon's book The Anabasis which records the failed invasion of Persia by Greek mercenary troops hired by Cyrus. Anabasis has the meaning of penetrating the interior of a country from its coast.

Maybe this explains my great-great grandfater, a Confederate Colonel. He was reputed to have never ordered a retreat, but was heard to call, "To the rear, charge!" a few times.

And no doubt he lead the charge, in the very van² of his troops. What a sterling record, with every charge successful -- else you would not be here to tell the tale.

So is the on-line source correct that anabasis can mean either military advance or military retreat? If so, I'd hate to have to rely on that word during a battle!

According to the OED, it only means "a military advance," though another definition was "The course of a disease from the commencement to the climax." It said the etymology is from Greek words meaning, "ascent, going up; to go or walk up; going, walk."

Xenophon's work covers both the entry into Persian territory and the subsequent need to retreat back to the sea, the sea. While the Greek word anabasis is applicable to the former part, the title of the book is Anabasis and so (perhaps only in English) is more associated with the march out and home.