The great orator Cicero, a leading figure in Roman political life during the middle decades of the last century B.C., is the first Roman author from whose works we can extract something like a comprehensive theory of warfare. It is essentially a Greek theory, but with some significant Roman contributions. The most complete version of it appears in the On Duties (De officiis), a summary of moral philosophy written at the end of Cicero’s life (circa 44 B.C.), based upon a similar treatise by Panaetius of Rhodes, the Greek neo-Stoic who had introduced Stoicism to the Roman aristocracy one hundred years before. Because of the great influence of this treatise on later Western ethical thought, the statements on warfare in On Duties merit full quotation.

The first office of justice is to keep one man from doing harm to another, unless provoked by wrong; and the next is to lead men to use common possessions for their common interests, private property for their own. There is, however, no such thing as private ownership established by nature, but property becomes private either through long occupancy (as in the case of those who long ago settled in unoccupied territory) or through conquest (as in the case of those who took it in war) or by due process of law, bargain, or purchase, or by allotment. (Duties 1.7.20-21 [trans. Walter Miller])