As foretold by God in Deuteronomy 17:14 and Deuteronomy 28:36, Israel would be ruled by a king. Only three kings ever ruled over all twelve tribes; Saul, David and Solomon.
David sang of the the covenant with Abraham.
“Remember his covenant forever, the word that he commanded, for a thousand generations, the covenant that he made with Abraham, his sworn promise to Isaac, which he confirmed to Jacob as a statute, to Israel as an everlasting covenant, saying, “To you I will give the land of Canaan, as your portion for an inheritance.” (1 Chronicles 16:15-18)
We also find the familiar theme of blessing for obedience or curses for disobedience when God speaks to Solomon.
“And as for you, if you will walk before me, as David your father walked, with integrity of heart and uprightness, doing according to all that I have commanded you, and keeping my statutes and my rules, then I will establish your royal throne over Israel forever, as I promised David your father, saying, ‘You shall not lack a man on the throne of Israel.’ But if you turn aside from following me, you or your children, and do not keep my commandments and my statutes that I have set before you, but go and serve other gods and worship them, then I will cut off Israel from the land that I have given them, and the house that I have consecrated for my name I will cast out of my sight, and Israel will become a proverb and a byword among all peoples. And this house will become a heap of ruins. Everyone passing by it will be astonished and will hiss, and they will say, ‘Why has the Lord done thus to this land and to this house?’ Then they will say, ‘Because they abandoned the Lord their God who brought their fathers out of the land of Egypt and laid hold on other gods and worshiped them and served them. Therefore the Lord has brought all this disaster on them.’” (1 Kings 9:4-9)
Solomon started pretty well, but was ultimately lured into idol worship.
“He had 700 wives, who were princesses, and 300 concubines. And his wives turned away his heart. For when Solomon was old his wives turned away his heart after other gods, and his heart was not wholly true to the Lord his God, as was the heart of David his father. For Solomon went after Ashtoreth the goddess of the Sidonians, and after Milcom the abomination of the Ammonites. So Solomon did what was evil in the sight of the Lord and did not wholly follow the Lord, as David his father had done. Then Solomon built a high place for Chemosh the abomination of Moab, and for Molech the abomination of the Ammonites, on the mountain east of Jerusalem. And so he did for all his foreign wives, who made offerings and sacrificed to their gods. And the Lord was angry with Solomon, because his heart had turned away from the Lord, the God of Israel, who had appeared to him twice and had commanded him concerning this thing, that he should not go after other gods. But he did not keep what the Lord commanded. Therefore the Lord said to Solomon, “Since this has been your practice and you have not kept my covenant and my statutes that I have commanded you, I will surely tear the kingdom from you and will give it to your servant. Yet for the sake of David your father I will not do it in your days, but I will tear it out of the hand of your son. However, I will not tear away all the kingdom, but I will give one tribe to your son, for the sake of David my servant and for the sake of Jerusalem that I have chosen.” (1 Kings 11:3-13)
The days of twelve unified tribes under one king came to and end after Solomon. The next king to rule over all twelve tribes would be a future descendent of David.