Surprisingly, the cause of ancient Israel splitting in two lies at the feet of King Solomon, the man blessed with unmatched wisdom and understanding at the start of his reign (1Kings 3:4 - 9, 2Chronicles 1:3 - 10)!
The root cause
Solomon was completely smitten and in love with foreign women (those who were not citizens in Israel). His first wife Naamah, married before he ascended the throne, was an Ammonite (1Kings 14:21) who gave birth to Rehoboam. His second wife, married soon after becoming ruler, was the daughter of Egypt's Pharaoh (1Kings 3:1). The king would ultimately mimic other rulers and marry a staggering number of women (700 wives, 300 concubines).
It wasn't that Solomon could not afford countless wives, as he was ridiculously wealthy (even more so than his righteous father David). The problem was that doing so sinned against God (Deuteronomy 17:18). The rulers over God's people were not to multiply women to themselves (verse 17).
Additionally, all men in Israel (including Solomon) were expressly forbidden to marry "strange" women who were idolaters or who did not choose to convert to worshipping the true God (as did Rahab the harlot and Ruth). This was because they would strongly influence their husbands to turn their hearts away from fully serving the Lord (Deuteronomy 7:1 - 4, 17:17).
The king, in order to please his many pagan wives, built various altars and "high places" for them to worship their false deities (1Kings 11:8, 2Kings 23:13, Nehemiah 13:26). Scripture also seems to indicate that Solomon in his old age also indulged, at what level is unknown, in worshipping the following pagan gods as well as the true God of Israel (1Kings 11:5 - 7, 10, 2Kings 23:13).
Worshipping pagan gods
Josephus, the first century Jewish historian, also asserts Solomon not only financially supported but also worshipped the foreign gods of his wives (Antiquities of the Jews, Book 8, Chapter 7, Section 5). These included Ashtoreth (aka Astarte), the goddess of the Sidonians. She was a deity of sex and fertility that involved licentious rites and worship of the heavens. He also worshipped Chemosh, the pagan sun god of war worshipped by the Moabites.
Lastly, Josephus contends Solomon worshipped Molech (Milcom), the evil deity of the Ammonites. One of the abominable rites of his worship was the sacrifice of children by burning them alive (known as "passing through the fire," see Leviticus 18:21, 20:1 - 5, 2Kings 23:10).
Solomon's indulgence in both pagan women and idolatry angered the Lord, especially when he had appeared twice to him (1Kings 11:9 - 10). The punishment for such grievous and public sins was to split the ancient kingdom by tearing ten of Israel's tribes away from Solomon's ruling dynasty (1Kings 11:11, 29 - 32).
The Lord also decided, for the sake of David, that the separation would take place during the reign of Solomon's son Rehoboam (1Kings 11:11 - 13). He also allowed several enemies to raise up civil unrest against him (1Kings 11:14 - 37), one of which was Jeroboam.
The foolish new king
Rehoboam, at the age of 41, assumes the throne of Israel in 930 B.C. He is approached, at his coronation, by Jeroboam and a delegation of Israel's elders. They propose that if the king lowers the incredibly heavy tax burden on them then they would loyally serve him as they did his father.
The king's older advisers, who counselled his father, suggested he listen to the people and lighten their tax load. The younger advisers, who grew up with Rehoboam, took for granted the power of the tribes to become independent if they so wished. They recommended he threaten them, like a dictator, with even higher taxes and harsher penalties as a means of quelling any thoughts of rebellion (1Kings 12).
The king's insulting and hostile answer to the elders of Israel solidified their resolve to leave the unified kingdom. Ten of the tribes formed the new Kingdom of Israel with Jeroboam as its first ruler. Rehoboam was left ruling over Judah, the tribe of Benjamin (whose territory included the capital city of Jerusalem) and the Levites who served God.
Solomon, reflecting on his life, stated that not one of his 1,000 wives proved to be virtuous or a positive influence on him (Ecclesiastes 7:26 - 28). When it came to love, he acted anything but wise. His indulgence in foreign women led him on a path that would ultimately punish all ancient Israel by causing the nation to split in two and robbing his descendants of a unified throne. God used the issue of taxation to complete his punishment of the foolish king (1Kings 12:15, 22 - 24).