Objection 1. It would seem that the judicial precepts regarding foreigners were not suitably framed. For Peter said (Acts 10:34-35): "In very deed I perceive that God is not a respecter of persons, but in every nation, he that feareth Him and worketh justice is acceptable to Him." But those who are acceptable to God should not be excluded from the Church of God. Therefore it is unsuitably commanded (Deuteronomy 23:3) that "the Ammonite and the Moabite, even after the tenth generation, shall not enter into the church of the Lord for ever": whereas, on the other hand, it is prescribed (Deuteronomy 23:7) to be observed with regard to certain other nations: "Thou shalt not abhor the Edomite, because he is thy brother; nor the Egyptian because thou wast a stranger in his land."
Objection 2. Further, we do not deserve to be punished for those things which are not in our power. But it is not in man's power to be an eunuch, or born of a prostitute. Therefore it is unsuitably commanded (Deuteronomy 23:1-2) that "an eunuch and one born of a prostitute shalt not enter into the church of the Lord."
Objection 3. Further, the Old Law mercifully forbade strangers to be molested: for it is written (Exodus 22:21): "Thou shalt not molest a stranger, nor afflict him; for yourselves also were strangers in the land of Egypt": and (Exodus 23:9): "Thou shalt not molest a stranger, for you know the hearts of strangers, for you also were strangers in the land of Egypt." But it is an affliction to be burdened with usury. Therefore the Law unsuitably permitted them (Deuteronomy 23:19-20) to lend money to the stranger for usury.
Objection 4. Further, men are much more akin to us than trees. But we should show greater care and love for these things that are nearest to us, according to Sirach 13:19: "Every beast loveth its like: so also every man him that is nearest to himself." Therefore the Lord unsuitably commanded (Deuteronomy 20:13-19) that all the inhabitants of a captured hostile city were to be slain, but that the fruit-trees should not be cut down.
Objection 5. Further, every one should prefer the common good of virtue to the good of the individual. But the common good is sought in a war which men fight against their enemies. Therefore it is unsuitably commanded (Deuteronomy 20:5-7) that certain men should be sent home, for instance a man that had built a new house, or who had planted a vineyard, or who had married a wife.
Objection 6. Further, no man should profit by his own fault. But it is a man's fault if he be timid or faint-hearted: since this is contrary to the virtue of fortitude. Therefore the timid and faint-hearted are unfittingly excused from the toil of battle (Deuteronomy 20:8).
On the contrary, Divine Wisdom declares (Proverbs 8:8): "All my words are just, there is nothing wicked nor perverse in them."
I answer that, Man's relations with foreigners are twofold: peaceful, and hostile: and in directing both kinds of relation the Law contained suitable precepts. For the Jews were offered three opportunities of peaceful relations with foreigners. First, when foreigners passed through their land as travelers. Secondly, when they came to dwell in their land as newcomers. And in both these respects the Law made kind provision in its precepts: for it is written (Exodus 22:21): "Thou shalt not molest a stranger [advenam]"; and again (Exodus 22:9): "Thou shalt not molest a stranger [peregrino]." Thirdly, when any foreigners wished to be admitted entirely to their fellowship and mode of worship. With regard to these a certain order was observed. For they were not at once admitted to citizenship: just as it was law with some nations that no one was deemed a citizen except after two or three generations, as the Philosopher says (Polit. iii, 1). The reason for this was that if foreigners were allowed to meddle with the affairs of a nation as soon as they settled down in its midst, many dangers might occur, since the foreigners not yet having the common good firmly at heart might attempt something hurtful to the people. Hence it was that the Law prescribed in respect of certain nations that had close relations with the Jews (viz., the Egyptians among whom they were born and educated, and the Idumeans, the children of Esau, Jacob's brother), that they should be admitted to the fellowship of the people after the third generation; whereas others (with whom their relations had been hostile, such as the Ammonites and Moabites) were never to be admitted to citizenship; while the Amalekites, who were yet more hostile to them, and had no fellowship of kindred with them, were to be held as foes in perpetuity: for it is written (Exodus 17:16): "The war of the Lord shall be against Amalec from generation to generation."
In like manner with regard to hostile relations with foreigners, the Law contained suitable precepts. For, in the first place, it commanded that war should be declared for a just cause: thus it is commanded (Deuteronomy 20:10) that when they advanced to besiege a city, they should at first make an offer of peace. Secondly, it enjoined that when once they had entered on a war they should undauntedly persevere in it, putting their trust in God. And in order that they might be the more heedful of this command, it ordered that on the approach of battle the priest should hearten them by promising them God's aid. Thirdly, it prescribed the removal of whatever might prove an obstacle to the fight, and that certain men, who might be in the way, should be sent home. Fourthly, it enjoined that they should use moderation in pursuing the advantage of victory, by sparing women and children, and by not cutting down fruit-trees of that country.
Reply to Objection 1. The Law excluded the men of no nation from the worship of God and from things pertaining to the welfare of the soul: for it is written (Exodus 12:48): "If any stranger be willing to dwell among you, and to keep the Phase of the Lord; all his males shall first be circumcised, and then shall he celebrate it according to the manner, and he shall be as that which is born in the land." But in temporal matters concerning the public life of the people, admission was not granted to everyone at once, for the reason given above: but to some, i.e. the Egyptians and Idumeans, in the third generation; while others were excluded in perpetuity, in detestation of their past offense, i.e. the peoples of Moab, Ammon, and Amalec. For just as one man is punished for a sin committed by him, in order that others seeing this may be deterred and refrain from sinning; so too may one nation or city be punished for a crime, that others may refrain from similar crimes.
Nevertheless it was possible by dispensation for a man to be admitted to citizenship on account of some act of virtue: thus it is related (Judith 14:6) that Achior, the captain of the children of Ammon, "was joined to the people of Israel, with all the succession of his kindred." The same applies to Ruth the Moabite who was "a virtuous woman" (Ruth 3:11): although it may be said that this prohibition regarded men and not women, who are not competent to be citizens absolutely speaking.
Reply to Objection 2. As the Philosopher says (Polit. iii, 3), a man is said to be a citizen in two ways: first, simply; secondly, in a restricted sense. A man is a citizen simply if he has all the rights of citizenship, for instance, the right of debating or voting in the popular assembly. On the other hand, any man may be called citizen, only in a restricted sense, if he dwells within the state, even common people or children or old men, who are not fit to enjoy power in matters pertaining to the common weal. For this reason bastards, by reason of their base origin, were excluded from the "ecclesia," i.e. from the popular assembly, down to the tenth generation. The same applies to eunuchs, who were not competent to receive the honor due to a father, especially among the Jews, where the divine worship was continued through carnal generation: for even among the heathens, those who had many children were marked with special honor, as the Philosopher remarks (Polit. ii, 6). Nevertheless, in matters pertaining to the grace of God, eunuchs were not discriminated from others, as neither were strangers, as already stated: for it is written (Isaiah 56:3): "Let not the son of the stranger that adhereth to the Lord speak, saying: The Lord will divide and separate me from His people. And let not the eunuch say: Behold I am a dry tree."
Reply to Objection 3. It was not the intention of the Law to sanction the acceptance of usury from strangers, but only to tolerate it on account of the proneness of the Jews to avarice; and in order to promote an amicable feeling towards those out of whom they made a profit.
Reply to Objection 4. A distinction was observed with regard to hostile cities. For some of them were far distant, and were not among those which had been promised to them. When they had taken these cities, they killed all the men who had fought against God's people; whereas the women and children were spared. But in the neighboring cities which had been promised to them, all were ordered to be slain, on account of their former crimes, to punish which God sent the Israelites as executor of Divine justice: for it is written (Deuteronomy 9:5) "because they have done wickedly, they are destroyed at thy coming in." The fruit-trees were commanded to be left untouched, for the use of the people themselves, to whom the city with its territory was destined to be subjected.
Reply to Objection 5. The builder of a new house, the planter of a vineyard, the newly married husband, were excluded from fighting, for two reasons. First, because man is wont to give all his affection to those things which he has lately acquired, or is on the point of having, and consequently he is apt to dread the loss of these above other things. Wherefore it was likely enough that on account of this affection they would fear death all the more, and be so much the less brave in battle. Secondly, because, as the Philosopher says (Phys. ii, 5), "it is a misfortune for a man if he is prevented from obtaining something good when it is within his grasp." And so lest the surviving relations should be the more grieved at the death of these men who had not entered into the possession of the good things prepared for them; and also lest the people should be horror-stricken at the sight of their misfortune: these men were taken away from the danger of death by being removed from the battle.
Reply to Objection 6. The timid were sent back home, not that they might be the gainers thereby; but lest the people might be the losers by their presence, since their timidity and flight might cause others to be afraid and run away.
Source: Summa Theologica (New Advent)