Objection 1. It would seem that robbery may be committed without sin. For spoils are taken by violence, and this seems to belong to the essence of robbery, according to what has been said (4). Now it is lawful to take spoils from the enemy; for Ambrose says (De Patriarch. 4 [De Abraham i, 3): "When the conqueror has taken possession of the spoils, military discipline demands that all should be reserved for the sovereign," in order, to wit, that he may distribute them. Therefore in certain cases robbery is lawful.
Objection 2. Further, it is lawful to take from a man what is not his. Now the things which unbelievers have are not theirs, for Augustine says (Ep. ad Vincent. Donat. xciii.): "You falsely call things your own, for you do not possess them justly, and according to the laws of earthly kings you are commanded to forfeit them." Therefore it seems that one may lawfully rob unbelievers.
Objection 3. Further, earthly princes violently extort many things from their subjects: and this seems to savor of robbery. Now it would seem a grievous matter to say that they sin in acting thus, for in that case nearly every prince would be damned. Therefore in some cases robbery is lawful.
On the contrary, Whatever is taken lawfully may be offered to God in sacrifice and oblation. Now this cannot be done with the proceeds of robbery, according to Isaiah 61:8, "I am the Lord that love judgment, and hate robbery in a holocaust." Therefore it is not lawful to take anything by robbery.
I answer that, Robbery implies a certain violence and coercion employed in taking unjustly from a man that which is his. Now in human society no man can exercise coercion except through public authority: and, consequently, if a private individual not having public authority takes another's property by violence, he acts unlawfully and commits a robbery, as burglars do. As regards princes, the public power is entrusted to them that they may be the guardians of justice: hence it is unlawful for them to use violence or coercion, save within the bounds of justice--either by fighting against the enemy, or against the citizens, by punishing evil-doers: and whatever is taken by violence of this kind is not the spoils of robbery, since it is not contrary to justice. On the other hand to take other people's property violently and against justice, in the exercise of public authority, is to act unlawfully and to be guilty of robbery; and whoever does so is bound to restitution.
Reply to Objection 1. A distinction must be made in the matter of spoils. For if they who take spoils from the enemy, are waging a just war, such things as they seize in the war become their own property. This is no robbery, so that they are not bound to restitution. Nevertheless even they who are engaged in a just war may sin in taking spoils through cupidity arising from an evil intention, if, to wit, they fight chiefly not for justice but for spoil. For Augustine says (De Verb. Dom. xix; Serm. lxxxii) that "it is a sin to fight for booty." If, however, those who take the spoil, are waging an unjust war, they are guilty of robbery, and are bound to restitution.
Reply to Objection 2. Unbelievers possess their goods unjustly in so far as they are ordered by the laws of earthly princes to forfeit those goods. Hence these may be taken violently from them, not by private but by public authority.
Reply to Objection 3. It is no robbery if princes exact from their subjects that which is due to them for the safe-guarding of the common good, even if they use violence in so doing: but if they extort something unduly by means of violence, it is robbery even as burglary is. Hence Augustine says (De Civ. Dei iv, 4): "If justice be disregarded, what is a king but a mighty robber? since what is a robber but a little king?" And it is written (Ezekiel 22:27): "Her princes in the midst of her, are like wolves ravening the prey." Wherefore they are bound to restitution, just as robbers are, and by so much do they sin more grievously than robbers, as their actions are fraught with greater and more universal danger to public justice whose wardens they are.
Source: Summa Theologica (New Advent)