I tried to search who this quote belongs (as it is good to always have a source as well) to and it seems this comes from a late Roman era (4th century) from Vegetius: De Re Militari (Lib . II . cap . 5.)
But in the original it is in the third person plural + few other differences:
Jurant autem milites, omnia se strenue facturos quæ præceperit imperator, nunquam deserturos (militiam) nec mortem recusaturos pro Romana republica.
Ah! I sort of thought (=on the second look) that the "forum" is some reference to the Forum Rōmānum or something, but ... ok xD I actually don't know how this section works at all, I thought people were just sending some quotations from the litterature or so