Now think of what David did. Prior to Saul and David being kings, the twelve tribes of Israel were independent governmental entities, even though all tribes regarded themselves as descendents of Abraham. What David did was to unite the people and all the tribes religiously (during David's reign the first Temple is started in Jerusalem), politically,
2 Samuel 5:1-6: "Then all the tribes of Israel went to David at Hebron and said to him `...the Lord promised you would lead his people and be their ruler.' So all the leaders of Israel came to King David at Hebron. He made a sacred alliance with them, they anointed him, and he became king of Israel. David was thirty years old when he became king, and he ruled for forty years. He ruled in Hebron over Judah for seven and a half years, and in Jerusalem over all Israel and Judah for thirty-three years."