The first 10 chapters of 1 Kings build an incredible resume for Solomon. However, in chapter 11 things take a turn as Solomon turns after the idols of his many wives. This is in clear violation of the agreement that Solomon has with the LORD.
To get the real tragedy of this it is important to understand a little geography. It is sad that Solomon builds temples to these false gods, it is a tragedy that he builds them overlooking the temple mount (1 Kings 11:7). This temple to a false god would be in plain sight of anyone going to offer sacrificed at the Temple on Jerusalem.
Also note that these false gods were often worshipped in the most terrible fasion including child sacrifices. That is human sacrifices of infant children. That Solomon would even build a temple to these false gods is a rejection of the LORD.
Here Solomon is guilty of many sins, the most important being idol worship. In Deuteronomy 17:14-20, There are certain commands given for the conduct if a king of Israel. You see Israel is not like the other nations. Other nations belong to their kings and the kings flaunt it. Israel belongs to the LORD. It is a God ruled government. As such the king is supposed to live much closer to the people than for himself.
Here we have seen Solomon living much for himself. He has violated the commands of God. He has gone against the law to marry many cannanite women and they have lead his heart astray.
Because Solomon has broken covenant with the Lord, the Lord raises up opposition for Solomon. A troublemaker arrises in the south, another in the north and finally God calls out a man from within Israel to cause problems for Solomon and eventually split the kingdom.
Application: How you start is not nearly important as how you finish. If Solomon the wisest and wealthiest man to ever live can blow it, so can I. He started out great, but lost focus in the end. Are you living in such a way as to finish well? Or are all your greatest days of trusting God behind you?