The Roman History · Amm. 14.10.7

When suddenly a guide well acquainted with the country arrived, and for a reward pointed out a ford by night, where the river could be crossed; and the army crossing at that point, while the enemy had their attention directed elsewhere, might without any one expecting such a step, have and waste the whole country, if a few men of the same nation to whom the higher posts in the Roman army were intrusted had not (as some people believe) informed their fellow-countrymen of the design by secret messengers.

Ammianus Marcellinus · translated by C. D. Yonge · 1911 reprint
C. D. Yonge translation (preface dated February 1862); G. Bell and Sons, 1911; Project Gutenberg #28587
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