Pliny the Younger (A.D. 62?c.A.D. 113). Letters. The Harvard Classics. 190914.
XXXIII. Trajan to Pliny
I HAVE received from Gabius Bassus the letter you mention, acquainting me that the number of soldiers I had ordered him was not sufficient; and for your information I have directed my answer to be hereunto annexed. It is very material to distinguish between what the exigency of affairs requires and what an ambitious desire of extending power may think necessary. As for ourselves, the public welfare must be our only guide: accordingly it is incumbent upon us to take all possible care that the soldiers shall not be absent from their standard.