Why did God cause the united Kingdom of Israel to divide into two separate nations?
The primary cause was Solomon turning to idolatry.
1 Kings 11:1-13 But king Solomon loved many strange women, together with the daughter of Pharaoh, women of the Moabites, Ammonites, Edomites, Zidonians, and Hittites; 2 Of the nations concerning which the LORD said unto the children of Israel, Ye shall not go in to them, neither shall they come in unto you: for surely they will turn away your heart after their gods: Solomon clave unto these in love. 3 And he had seven hundred wives, princesses, and three hundred concubines: and his wives turned away his heart. 4 For it came to pass, when Solomon was old, that his wives turned away his heart after other gods: and his heart was not perfect with the LORD his God, as was the heart of David his father. 5 For Solomon went after Ashtoreth the goddess of the Zidonians, and after Milcom the abomination of the Ammonites. 6 And Solomon did evil in the sight of the LORD, and went not fully after the LORD, as did David his father. 7 Then did Solomon build an high place for Chemosh, the abomination of Moab, in the hill that is before Jerusalem, and for Molech, the abomination of the children of Ammon. 8 And likewise did he for all his strange wives, which burnt incense and sacrificed unto their gods. 9 And the LORD was angry with Solomon, because his heart was turned from the LORD God of Israel, which had appeared unto him twice, 10 And had commanded him concerning this thing, that he should not go after other gods: but he kept not that which the LORD commanded. 11 Wherefore the LORD said unto Solomon, Forasmuch as this is done of thee, and thou hast not kept my covenant and my statutes, which I have commanded thee, I will surely rend the kingdom from thee, and will give it to thy servant. 12 Notwithstanding in thy days I will not do it for David thy father’s sake: but I will rend it out of the hand of thy son. 13 Howbeit I will not rend away all the kingdom; but will give one tribe to thy son for David my servant’s sake, and for Jerusalem’s sake which I have chosen.
God promised ten tribes to a man of Ephraim called Jeroboam.
1 Kings 11:28 -39 And the man Jeroboam was a mighty man of valour: and Solomon seeing the young man that he was industrious, he made him ruler over all the charge of the house of Joseph. 29 And it came to pass at that time when Jeroboam went out of Jerusalem, that the prophet Ahijah the Shilonite found him in the way; and he had clad himself with a new garment; and they two were alone in the field: 30 And Ahijah caught the new garment that was on him, and rent it in twelve pieces: 31 And he said to Jeroboam, Take thee ten pieces: for thus saith the LORD, the God of Israel, Behold, I will rend the kingdom out of the hand of Solomon, and will give ten tribes to thee: 32 (But he shall have one tribe for my servant David’s sake, and for Jerusalem’s sake, the city which I have chosen out of all the tribes of Israel) 33 Because that they have forsaken me, and have worshipped Ashtoreth the goddess of the Zidonians, Chemosh the god of the Moabites, and Milcom the god of the children of Ammon, and have not walked in my ways, to do that which is right in mine eyes, and to keep my statutes and my judgments, as did David his father. 34 Howbeit I will not take the whole kingdom out of his hand: but I will make him prince all the days of his life for David my servant’s sake, whom I chose, because he kept my commandments and my statutes: 35 But I will take the kingdom out of his son’s hand, and will give it unto thee, even ten tribes. 36 And unto his son will I give one tribe, that David my servant may have a light alway before me in Jerusalem, the city which I have chosen me to put my name there. 37 And I will take thee, and thou shalt reign according to all that thy soul desireth, and shalt be king over Israel. 38 And it shall be, if thou wilt hearken unto all that I command thee, and wilt walk in my ways, and do that is right in my sight, to keep my statutes and my commandments, as David my servant did; that I will be with thee, and build thee a sure house, as I built for David, and will give Israel unto thee. 39 And I will for this afflict the seed of David, but not for ever.
The secondary cause was Rehoboam’s foolishness.
1 Kings 12:1-24 And Rehoboam went to Shechem: for all Israel were come to Shechem to make him king. 2 And it came to pass, when Jeroboam the son of Nebat, who was yet in Egypt, heard of it, (for he was fled from the presence of king Solomon, and Jeroboam dwelt in Egypt;) 3 That they sent and called him. And Jeroboam and all the congregation of Israel came, and spake unto Rehoboam, saying, 4 Thy father made our yoke grievous: now therefore make thou the grievous service of thy father, and his heavy yoke which he put upon us, lighter, and we will serve thee. 5 And he said unto them, Depart yet for three days, then come again to me. And the people departed. 6 And king Rehoboam consulted with the old men, that stood before Solomon his father while he yet lived, and said, How do ye advise that I may answer this people? 7 And they spake unto him, saying, If thou wilt be a servant unto this people this day, and wilt serve them, and answer them, and speak good words to them, then they will be thy servants for ever. 8 But he forsook the counsel of the old men, which they had given him, and consulted with the young men that were grown up with him, and which stood before him: 9 And he said unto them, What counsel give ye that we may answer this people, who have spoken to me, saying, Make the yoke which thy father did put upon us lighter? 10 And the young men that were grown up with him spake unto him, saying, Thus shalt thou speak unto this people that spake unto thee, saying, Thy father made our yoke heavy, but make thou it lighter unto us; thus shalt thou say unto them, My little finger shall be thicker than my father’s loins. 11 And now whereas my father did lade you with a heavy yoke, I will add to your yoke: my father hath chastised you with whips, but I will chastise you with scorpions. 12 So Jeroboam and all the people came to Rehoboam the third day, as the king had appointed, saying, Come to me again the third day. 13 And the king answered the people roughly, and forsook the old men’s counsel that they gave him; 14 And spake to them after the counsel of the young men, saying, My father made your yoke heavy, and I will add to your yoke: my father also chastised you with whips, but I will chastise you with scorpions. 15 Wherefore the king hearkened not unto the people; for the cause was from the LORD, that he might perform his saying, which the LORD spake by Ahijah the Shilonite unto Jeroboam the son of Nebat. 16 So when all Israel saw that the king hearkened not unto them, the people answered the king, saying, What portion have we in David? neither have we inheritance in the son of Jesse: to your tents, O Israel: now see to thine own house, David. So Israel departed unto their tents. 17 But as for the children of Israel which dwelt in the cities of Judah, Rehoboam reigned over them. 18 Then king Rehoboam sent Adoram, who was over the tribute; and all Israel stoned him with stones, that he died. Therefore king Rehoboam made speed to get him up to his chariot, to flee to Jerusalem. 19 So Israel rebelled against the house of David unto this day. 20 And it came to pass, when all Israel heard that Jeroboam was come again, that they sent and called him unto the congregation, and made him king over all Israel: there was none that followed the house of David, but the tribe of Judah only. 21 And when Rehoboam was come to Jerusalem, he assembled all the house of Judah, with the tribe of Benjamin, an hundred and fourscore thousand chosen men, which were warriors, to fight against the house of Israel, to bring the kingdom again to Rehoboam the son of Solomon. 22 But the word of God came unto Shemaiah the man of God, saying, 23 Speak unto Rehoboam, the son of Solomon, king of Judah, and unto all the house of Judah and Benjamin, and to the remnant of the people, saying, 24 Thus saith the LORD, Ye shall not go up, nor fight against your brethren the children of Israel: return every man to his house; for this thing is from me. They hearkened therefore to the word of the LORD, and returned to depart, according to the word of the LORD.
The ten-tribe kingdom of Israel turned to idolatry and was eventually removed from the land by the Assyrians. The two-tribe kingdom of Judah went into temporary captivity to Babylon and was finally removed from the land by the Romans. God has promised that all the tribes will be restored as one nation in the land promised to Jacob.
Ezekiel 37:16-28 Moreover, thou son of man, take thee one stick, and write upon it, For Judah, and for the children of Israel his companions: then take another stick, and write upon it, For Joseph, the stick of Ephraim, and for all the house of Israel his companions: 17 And join them one to another into one stick; and they shall become one in thine hand. 18 And when the children of thy people shall speak unto thee, saying, Wilt thou not shew us what thou meanest by these? 19 Say unto them, Thus saith the Lord GOD; Behold, I will take the stick of Joseph, which is in the hand of Ephraim, and the tribes of Israel his fellows, and will put them with him, even with the stick of Judah, and make them one stick, and they shall be one in mine hand. 20 And the sticks whereon thou writest shall be in thine hand before their eyes. 21 And say unto them, Thus saith the Lord GOD; Behold, I will take the children of Israel from among the heathen, whither they be gone, and will gather them on every side, and bring them into their own land: 22 And I will make them one nation in the land upon the mountains of Israel; and one king shall be king to them all: and they shall be no more two nations, neither shall they be divided into two kingdoms any more at all: 23 Neither shall they defile themselves any more with their idols, nor with their detestable things, nor with any of their transgressions: but I will save them out of all their dwellingplaces, wherein they have sinned, and will cleanse them: so shall they be my people, and I will be their God. 24 And David my servant shall be king over them; and they all shall have one shepherd: they shall also walk in my judgments, and observe my statutes, and do them. 25 And they shall dwell in the land that I have given unto Jacob my servant, wherein your fathers have dwelt; and they shall dwell therein, even they, and their children, and their children’s children for ever: and my servant David shall be their prince for ever. 26 Moreover I will make a covenant of peace with them; it shall be an everlasting covenant with them: and I will place them, and multiply them, and will set my sanctuary in the midst of them for evermore. 27 My tabernacle also shall be with them: yea, I will be their God, and they shall be my people. 28 And the heathen shall know that I the LORD do sanctify Israel, when my sanctuary shall be in the midst of them for evermore.
This will happen when Christ returns. Jesus will restore the nation and rule the earth from the throne of David.
Luke 1:32-33 He shall be great, and shall be called the Son of the Highest: and the Lord God shall give unto him the throne of his father David: 33 And he shall reign over the house of Jacob for ever; and of his kingdom there shall be no end.
Isaiah 2:1-4 The word that Isaiah the son of Amoz saw concerning Judah and Jerusalem. 2 And it shall come to pass in the last days, that the mountain of the LORD’S house shall be established in the top of the mountains, and shall be exalted above the hills; and all nations shall flow unto it. 3 And many people shall go and say, Come ye, and let us go up to the mountain of the LORD, to the house of the God of Jacob; and he will teach us of his ways, and we will walk in his paths: for out of Zion shall go forth the law, and the word of the LORD from Jerusalem. 4 And he shall judge among the nations, and shall rebuke many people: and they shall beat their swords into plowshares, and their spears into pruninghooks: nation shall not lift up sword against nation, neither shall they learn war any more.
The future resettlement of the tribes is described in Ezekiel 48.
I hope you find this helpful.