Objection 1. It would seem that the Old Law made unfitting precepts concerning rulers. Because, as the Philosopher says (Polit. iii, 4), "the ordering of the people depends mostly on the chief ruler." But the Law contains no precept relating to the institution of the chief ruler; and yet we find therein prescriptions concerning the inferior rulers: firstly (Exodus 18:21): "Provide out of all the people wise [Vulgate: 'able'] men," etc.; again (Numbers 11:16): "Gather unto Me seventy men of the ancients of Israel"; and again (Deuteronomy 1:13): "Let Me have from among you wise and understanding men," etc. Therefore the Law provided insufficiently in regard to the rulers of the people.
Objection 2. Further, "The best gives of the best," as Plato states (Tim. ii). Now the best ordering of a state or of any nation is to be ruled by a king: because this kind of government approaches nearest in resemblance to the Divine government, whereby God rules the world from the beginning. Therefore the Law should have set a king over the people, and they should not have been allowed a choice in the matter, as indeed they were allowed (Deuteronomy 17:14-15): "When thou... shalt say: I will set a king over me... thou shalt set him," etc.
Objection 3. Further, according to Matthew 12:25: "Every kingdom divided against itself shall be made desolate": a saying which was verified in the Jewish people, whose destruction was brought about by the division of the kingdom. But the Law should aim chiefly at things pertaining to the general well-being of the people. Therefore it should have forbidden the kingdom to be divided under two kings: nor should this have been introduced even by Divine authority; as we read of its being introduced by the authority of the prophet Ahias the Silonite (1 Kings 11:29, seqq.).
Objection 4. Further, just as priests are instituted for the benefit of the people in things concerning God, as stated in Hebrews 5:1; so are rulers set up for the benefit of the people in human affairs. But certain things were allotted as a means of livelihood for the priests and Levites of the Law: such as the tithes and first-fruits, and many like things. Therefore in like manner certain things should have been determined for the livelihood of the rulers of the people: the more that they were forbidden to accept presents, as is clearly stated in Exodus 23:8: "You shall not [Vulgate: 'Neither shalt thou'] take bribes, which even blind the wise, and pervert the words of the just."
Objection 5. Further, as a kingdom is the best form of government, so is tyranny the most corrupt. But when the Lord appointed the king, He established a tyrannical law; for it is written (1 Samuel 8:11): "This will be the right of the king, that shall reign over you: He will take your sons," etc. Therefore the Law made unfitting provision with regard to the institution of rulers.
On the contrary, The people of Israel is commended for the beauty of its order (Numbers 24:5): "How beautiful are thy tabernacles, O Jacob, and thy tents." But the beautiful ordering of a people depends on the right establishment of its rulers. Therefore the Law made right provision for the people with regard to its rulers.
I answer that, Two points are to be observed concerning the right ordering of rulers in a state or nation. One is that all should take some share in the government: for this form of constitution ensures peace among the people, commends itself to all, and is most enduring, as stated in Polit. ii, 6. The other point is to be observed in respect of the kinds of government, or the different ways in which the constitutions are established. For whereas these differ in kind, as the Philosopher states (Polit. iii, 5), nevertheless the first place is held by the "kingdom," where the power of government is vested in one; and "aristocracy," which signifies government by the best, where the power of government is vested in a few. Accordingly, the best form of government is in a state or kingdom, where one is given the power to preside over all; while under him are others having governing powers: and yet a government of this kind is shared by all, both because all are eligible to govern, and because the rules are chosen by all. For this is the best form of polity, being partly kingdom, since there is one at the head of all; partly aristocracy, in so far as a number of persons are set in authority; partly democracy, i.e. government by the people, in so far as the rulers can be chosen from the people, and the people have the right to choose their rulers.
Such was the form of government established by the Divine Law. For Moses and his successors governed the people in such a way that each of them was ruler over all; so that there was a kind of kingdom. Moreover, seventy-two men were chosen, who were elders in virtue: for it is written (Deuteronomy 1:15): "I took out of your tribes wise and honorable, and appointed them rulers": so that there was an element of aristocracy. But it was a democratical government in so far as the rulers were chosen from all the people; for it is written (Exodus 18:21): "Provide out of all the people wise [Vulgate: 'able'] men," etc.; and, again, in so far as they were chosen by the people; wherefore it is written (Deuteronomy 1:13): "Let me have from among you wise [Vulgate: 'able'] men," etc. Consequently it is evident that the ordering of the rulers was well provided for by the Law.
Reply to Objection 1. This people was governed under the special care of God: wherefore it is written (Deuteronomy 7:6): "The Lord thy God hath chosen thee to be His peculiar people": and this is why the Lord reserved to Himself the institution of the chief ruler. For this too did Moses pray (Numbers 27:16): "May the Lord the God of the spirits of all the flesh provide a man, that may be over this multitude." Thus by God's orders Josue was set at the head in place of Moses; and we read about each of the judges who succeeded Josue that God "raised... up a saviour" for the people, and that "the spirit of the Lord was" in them (Judges 3:9-15). Hence the Lord did not leave the choice of a king to the people; but reserved this to Himself, as appears from Deuteronomy 17:15: "Thou shalt set him whom the Lord thy God shall choose."
Reply to Objection 2. A kingdom is the best form of government of the people, so long as it is not corrupt. But since the power granted to a king is so great, it easily degenerates into tyranny, unless he to whom this power is given be a very virtuous man: for it is only the virtuous man that conducts himself well in the midst of prosperity, as the Philosopher observes (Ethic. iv, 3). Now perfect virtue is to be found in few: and especially were the Jews inclined to cruelty and avarice, which vices above all turn men into tyrants. Hence from the very first the Lord did not set up the kingly authority with full power, but gave them judges and governors to rule them. But afterwards when the people asked Him to do so, being indignant with them, so to speak, He granted them a king, as is clear from His words to Samuel (1 Samuel 8:7): "They have not rejected thee, but Me, that I should not reign over them."
Nevertheless, as regards the appointment of a king, He did establish the manner of election from the very beginning (Deuteronomy 17:14, seqq.): and then He determined two points: first, that in choosing a king they should wait for the Lord's decision; and that they should not make a man of another nation king, because such kings are wont to take little interest in the people they are set over, and consequently to have no care for their welfare: secondly, He prescribed how the king after his appointment should behave, in regard to himself; namely, that he should not accumulate chariots and horses, nor wives, nor immense wealth: because through craving for such things princes become tyrants and forsake justice. He also appointed the manner in which they were to conduct themselves towards God: namely, that they should continually read and ponder on God's Law, and should ever fear and obey God. Moreover, He decided how they should behave towards their subjects: namely, that they should not proudly despise them, or ill-treat them, and that they should not depart from the paths of justice.
Reply to Objection 3. The division of the kingdom, and a number of kings, was rather a punishment inflicted on that people for their many dissensions, specially against the just rule of David, than a benefit conferred on them for their profit. Hence it is written (Hosea 13:11): "I will give thee a king in My wrath"; and (Hosea 8:4): "They have reigned, but not by Me: they have been princes, and I knew not."
Reply to Objection 4. The priestly office was bequeathed by succession from father to son: and this, in order that it might be held in greater respect, if not any man from the people could become a priest: since honor was given to them out of reverence for the divine worship. Hence it was necessary to put aside certain things for them both as to tithes and as to first-fruits, and, again, as to oblations and sacrifices, that they might be afforded a means of livelihood. On the other hand, the rulers, as stated above, were chosen from the whole people; wherefore they had their own possessions, from which to derive a living: and so much the more, since the Lord forbade even a king to have superabundant wealth to make too much show of magnificence: both because he could scarcely avoid the excesses of pride and tyranny, arising from such things, and because, if the rulers were not very rich, and if their office involved much work and anxiety, it would not tempt the ambition of the common people; and would not become an occasion of sedition.
Reply to Objection 5. That right was not given to the king by Divine institution: rather was it foretold that kings would usurp that right, by framing unjust laws, and by degenerating into tyrants who preyed on their subjects. This is clear from the context that follows: "And you shall be his slaves [Douay: 'servants']": which is significative of tyranny, since a tyrant rules is subjects as though they were his slaves. Hence Samuel spoke these words to deter them from asking for a king; since the narrative continues: "But the people would not hear the voice of Samuel." It may happen, however, that even a good king, without being a tyrant, may take away the sons, and make them tribunes and centurions; and may take many things from his subjects in order to secure the common weal.
Source: Summa Theologica (New Advent)