Given the size and scope of the empire he inherited from his father David, Solomon no doubt needed means for transportation and trade. Beyond this, horses were prime war materiel in those days, particularly for pulling chariots, so multiplying horses can indicate territorial aggression and a warlike spirit. Most importantly, it can show a lack of faith in God and too great a faith in armies.
I Kings 10:26, 28 says that Solomon had thousands of horses imported from Egypt. The next verse reveals further proof of his departure from these royal guidelines: He also imported chariots and sold horses and chariots to other nations. What surfaces here is nothing more than a tenth-century BC arms race! Solomon armed the Hittites and Syria, providing them with the means to attack Israel and Judah in later years (I Kings 11:23-25; 20:1; II Chronicles 22:5). In so doing, he violated one of the "smaller" precepts of God's law, first given to Israel 450 years earlier.
Martin G. Collins
The Enduring Results of Compromise
God was planning that Israel would have a king, so He laid down these regulations to show how He expected the king to conduct himself within the office. These regulations are designed to ensure that the king does not overly elevate himself above the people and rule as an autocratic despot. Instead, he is to be thoroughly familiar with and guided by the attitudes and laws of God. He must comprehensively know that his own nature is just like those he serves and be humbled.
John W. Ritenbaugh
Where Is the Beast? (Part 7)
Other Forerunner Commentary entries containing Deuteronomy 17:16:
1 Samuel 8:7-8
1 Samuel 8:7-9
1 Samuel 8:7-8
1 Samuel :
1 Kings 3:5-10
1 Kings 11:42
2 Chronicles 9:22-28