King Solomon broke all of the restrictions on monarchs in Deuteronomy 17:16-17. The Kings of Israel were not supposed to buy horses from Egypt, accumulate a bunch of wives (especially pagan Gentile women), or accumulate vast amounts of personal wealth for themselves.
Solomon bought 1,400 chariots and 12,000 horses from Egypt and Cilicia (1 Kings 10:26-29). Its likely he already had a business contract with Egypt because the first wife of Solomon that is mentioned is the Pharaoh’s daughter (1 Kings 9:16). Speaking of wives he had 700 wives and 300 concubines, and since they were foreign women who worshipped idols this was not good because they convinced Solomon to build shrines for false gods, turning him away from his covenant with his God (1 Kings 11:1-8).
Lastly, he accumulated a lot of wealth in gold and silver, and cedarwood (1 Kings 10:14-29). A lot of that wealth came from selling goods to merchants and traders, and kings in Arabia, charging wealthy people to come to hear his wisdom (v25) or reselling chariots from Egypt to the Hittites (v29). He also made money from heavy taxation and conscripting slave labor from various foreigners (2 Chronicles 2:17-18 and 2 Chron 8:7-10). When Solomon died the people went to his son Rehoboam to receive some relief from the heavy taxes but Rehoboam didn’t budge, and this is what lead to Jeroboam’s rebellion (1 Kings 12:1-20 and 2 Chronicles 10). This caused the Kingdom to split between the north (Israel) and the south (Judah).
I knew about the wives, but when reading I wondered about the other stuff, and I did a word search for horses and did some research on his wealth and I found these other things that he did. Not only that but the next verses reveal why he should have known better.
Deuteronomy 17:18 (NLT) “When he sits on the throne as king, he must copy for himself this body of instruction on a scroll in the presence of the Levitical priests. 19 He must always keep that copy with him and read it daily as long as he lives. That way he will learn to fear the Lord his God by obeying all the terms of these instructions and decrees. 20 This regular reading will prevent him from becoming proud and acting as if he is above his fellow citizens. It will also prevent him from turning away from these commands in the smallest way. And it will ensure that he and his descendants will reign for many generations in Israel.