Albert Barnes Commentary Deuteronomy 17:16

"Only he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he may multiply horses; forasmuch as Jehovah hath said unto you, Ye shall henceforth return no more that way." — Deuteronomy 17:16 (ASV)

The horse was not used in ancient times in the East for purposes of agriculture or travel, but ordinarily for war only. It appears constantly in Scripture as the symbol and embodiment of fleshly strength and the might of the creature (Psalms 33:16–17; Psalms 147:10; Job 39:19 and following), and is sometimes significantly spoken of simply as “the strong one” . Therefore, the spirit of the prohibition is that the king of Israel must not, like other earthly potentates, put his trust in costly and formidable preparations for war .

Egypt was the principal source from which the nations of western Asia drew their supplies of this animal (compare Exodus 14:5 and following; 1 Kings 10:28–29; 2 Kings 7:6); but contact, commerce, or alliance that would “cause the people to return to Egypt” would be to reverse that great and beneficent wonderwork of God which inaugurated the Mosaic covenant, the deliverance from the bondage of Egypt; and to deliberately bring about what God threatened (Deuteronomy 28:68) as the most severe punishment for Israel’s sin.