What juster reason is there for the waging of war than to repel slavery? a condition in which, though your master may not be oppressive, yet it is a wretched thing he should have the power to be so if he will.

[Quae causa iustior est belli gerendi quam servitutis depulsio? in qua etiamsi not sit molestus dominus, tamen est miserrimum posse, se velit.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Philippics [Philippicae; Antonian Orations], No. 8, ch. 4 / sec. 12 (8.4/8.12) (43-02-03 BC) [tr. Ker (Loeb) (1926)]
    (Source)

(Source (Latin)). Other translations:

What juster cause is there for waging war than the wish to repel slavery? in which, even if one's master be not tyrannical, yet it is a most miserable thing that he should be able to be so if he chooses.
[tr. Yonge (1903)]

Is there any better reason for waging war than to ward off slavery? In slavery, even if the master is not oppressive, the sorry thing still is that he can be if he wishes.
[tr. Manuwald (2007)]

What cause for war is more just than the repulsion of slavery? even under a benign master, it is miserable that he has the power, if he wants to use it.
[tr. Wiseman]