Albert Barnes Commentary Deuteronomy 17:15

"thou shalt surely set him king over thee, whom Jehovah thy God shall choose: one from among thy brethren shalt thou set king over thee; thou mayest not put a foreigner over thee, who is not thy brother." — Deuteronomy 17:15 (ASV)

The king, like the judges and officers , is to be chosen by the people; but their choice is to be in accordance with the will of God, and to be made from among “their brethren.” Compare 1 Samuel 9:15; 1 Samuel 10:24; 1 Samuel 16:1; 1 Kings 19:16.

Thou mayest not set a stranger over thee—The Jews extended this prohibition to all offices whatsoever ; and naturally attached the greatest importance to it: which explains the significance of the question proposed to our Lord, “Is it lawful to give tribute to Caesar?” (Matthew 22:17). A Gentile head for the Jewish people, whom the Law principally aimed to keep special and distinct from others, was an anomaly.