
“War is just when it is necessary; arms are permissible when there is no hope except in arms.”
This is a quotation of Titus Livius IX:1 iustum enim est bellum quibus necessarium, et pia arma ubi nulla in armis spes est) that Machiavelli uses in Ch. 24 of Discourses on Livy; Machiavelli similarly writes that "The justice of the cause is conspicuous; for that war is just which is necessary, and those arms are sacred from which we derive our only hope." (The Prince, Ch. 26)
Misattributed