Adam Clarke on 1 Kings 12:20

Made him king over all Israel - What is called Israel here, was ten-twelfths of the whole nation; and had they a right to call another person to the throne? They had not, - they had neither legal nor constitutional right. Jeroboam was not of the blood royal; he had no affinity to the kingdom. Nothing could justify this act, but the just judgment of God. God thus punished a disobedient and gainsaying people; and especially Solomon's family, whose sins against the Lord were of no ordinary magnitude.