Deuteronomy 17:16
But he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he should multiply horses: forasmuch as the LORD hath said unto you, Ye shall henceforth return no more that way. But he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he should multiply horses: forasmuch as the LORD hath said unto you, Ye shall henceforth return no more that way.
This command belongs to instructions given in foresight of Israel's future desire and request for a king, foretelling a monarchical form of government. Foretelling kingship in this way did not mean that monarchy was God's will, for when the people refused His wise counsel He permitted them to take their course and had Samuel grant their request while warning them of His disapproval and its consequences. Provision for kingship did not consent to any change in the principles of the state, for the government was still to be administered in God's name and authority, rulers were only to enforce the laws God had given with no authority to legislate, the king was to act as vicegerent of the Most High, God was to be recognized as head of the nation, and His law enforced as supreme law of the land. God reserved to Himself the right to choose the king, and Israel at first honored this by leaving the selection entirely to Him.
The verse forbids the king to build up a large supply of horses for himself or to send the people back to Egypt to obtain them, because the Lord had said they shall return no more that way. The restriction was given to Israel's rulers to keep them faithful to God's law and as a safeguard against dependence on human power. Solomon disregarded the command by bringing in horses from Egypt and other lands and amassing horses, chariots, and horsemen. In his choice of military equipment he followed human judgment instead of God's word.