The period of disunity of the biblical Israelite tribes is known as that of the Judges (ca. 15-11 centuries BCE). This prolonged time is characterized by the absence of a territorial unity, a centralized government, and a unified religious worship. It is considered to be a turbulent time when outbursts of violence caused by tribalism and external aggression were common. In the Book of Judges, a frequent refrain is this, “In those days there was no king in Israel.” (Judges 17:6; 18:1; 19:1) The book, repeating this refrain, ends with the sad verdict, “every man did that which was right in his own eyes.” (Judges 21:25)
The Israelite tribes at the times of the Judges formed a loose confederation which showed itself weak in the face of the frictions between the competing tribes and the enemies’ attacks. The tribes had their own portions of the land allotted to them at the conquest of Canaan. But due to the disparity in the population numbers and in the relative prominence of the various tribes, Israel was not a unified state.
A whole tribe of Benjamin was nearly destroyed by the rest of Israel (Judges 20:46), and on another occasion, many of the Ephraim tribe were massacred (Judges 12:6).
Contrary to Mosaic constitution, there emerged several worship sites. This did not promote unity either but rather led to aberrations in the administration of religious duties.
Judges were leaders raised by God in various tribes to defend Israel from the oppressors (Moabites, Ammonites, Edomites, Canaanites, Midianites etc.) But their charismatic leadership was an incomplete and only temporary solution to the above problems.
A sense of the growing need in a centralized government was felt towards the end of the Judges era, when the people asked the last of them, Samuel, to make them a king to judge them like all the nations (1 Samuel 8:5). Samuel chose Saul to be such a king over all of the tribes of Israel.
Saul began to deliver Israel from the long oppression of the Philistines. Though with some disfavor from Samuel (and God), he received the royal authority over the whole of the nation that no judge had had before. His successor king David completed the task of delivering the people and conquering the enemies. In fact, he greatly expanded the territory of the kingdom. Upon capturing Jebus, a Canaanite city, he transferred all of the administration and religious worship there, turning it into the capital of unified Israel and renaming it Jerusalem.
So, for Israel at the times of the Judges, the unifying factors were the struggle against the common enemies, the centralization of the religious worship and government, and the elevation of the rule from chiefdom to monarchy.