Why Not to Reshelve Books at the Library

I thought I knew why, but I was wrong. Here's the real scoop.

ID, Please?

The prologue to Andre Norton's Star Rangers - one of the first SF books I read, and one with a lasting impact on me - begins with these words:

There is an old legend concerning a Roman Emperor, who, to show his power, singled out the Tribune of a loyal legion and commanded that he march his men across Asia to the end of the world. And so a thousand men vanished into the hinterland of the largest continent, to be swallowed up forever. On some unknown battlefield the last handful of survivors must have formed a square which was overwhelmed by a barbarian charge. And their eagle may have stood lonely and tarnished in a horsehide tent for a generation thereafter. But it may be guessed, by those who know of the pride of these men in their corps and tradition, that they did march east as long as one still remained on his feet.

Does anyone know what Norton is referring to here?