Deuteronomy 17:15

you shall inquire for instruction before the Lord and afterward appoint the king over you: but it will not be lawful to set over you a foreign man who is not of your brethren.

you shall set i.e., it is permissible [the halakha [Sanhedrin 20b] is that it is mandatory — Translator] to do so,

so long as the monarch is chosen by a prophet, or through the use of the Urim — the point being that you yourselves may not choose the king.

you cannot (i.e., you may not)

enthrone your brother if he be not an Israelite (e.g., an Edomite, even though he is called “your brother” [23: 8]). The reason for the law about the horses is that in those times horses were imported from Egypt. The evidence is the passage “a chariot going out of Egypt…” [I Kings 10: 29], which occurred during Solomonic times. Because of Solomon’s need for their horses, he caused the people to sin and to return to Egypt. Obviously, it is a disgrace to return to Egypt after God has redeemed us from there (Uriah the prophet fled there only because his life was in danger [Jeremiah 26: 21]; and Jeremiah went there against his will [Jeremiah 43: 5-7]).

THOU SHALT BY ALL MEANS SET HIM KING OVER THEE, WHOM THE ETERNAL THY G-D SHALL CHOOSE. In the opinion of the commentators this means that he is to be chosen according to the word of a prophet or the judgment of the Urim. Scripture is thus commanding: “You shall set up a king over you, whom the Eternal thy G-d shall choose, for it is He Who will choose and not you.” And in the Sifre it is stated: 157. “Thou shalt by all means set him king over thee. If he dies, appoint another one in his place. A king — but not a queen. Whom the Eternal thy G-d shall choose — according to the word of the prophet.” But, if so, what sense is there to warn [in the verse before us], thou mayest not put a foreigner over thee, who is not thy brother, when G-d would not choose a foreigner? However, in the opinion of our Rabbis there is in this verse a hidden condition, stating: “you shall by all means set a king over you, whom G-d shall choose provided you can do so — if G-d will answer you through prophets; but a foreigner you may never appoint as king over you.”
And in line with the plain meaning of Scripture they have said, “thou shalt by all means set him king over thee, whom the Eternal thy G-d will choose and not him whom the Eternal thy G-d hateth, for He will choose an Israelite to be king over His chosen ones and not from among the other peoples.” In my opinion the simple meaning of the expression whom He will choose is that every ruler over people receives his position from G-d, as it is written until thou wilt know that the Most High ruleth in the kingdom of men, and giveth it to whomsoever He will. Similarly the Rabbis have said: “Even a superintendent of the well is appointed in heaven.” Thus the verse is stating: “thou shalt by all means set him king over thee — whoever it be decreed by Heaven that he is to reign, even if he be of the smallest of the tribes of Israel and of the least of all the families of the tribe — but you are never to invest a foreigner with regal power.” Similarly, by way of the plain meaning of Scripture, the expression the place which the Eternal thy G-d shall choose means “wherever G-d’s Sanctuary shall be built, was all the Will of G-d.”

איש נכרי, who is to lead you in battle;

אשר לא אחיך הוא, for he may seduce you to serve idols.