Charles Ellicott Commentary Genesis 49:28

"All these are the twelve tribes of Israel: and this is it that their father spake unto them and blessed them; every one according to his blessing he blessed them." — Genesis 49:28 (ASV)

These are the twelve tribes. —As we have seen in the case of Dan, Jacob had the further objective of forming his descendants into twelve separate communities, which were, like the States in America, each to be independent and have its own tribal government. From this position, Levi was naturally excluded when selected for the priesthood, and room was thus made for the bestowal of two of these communities upon the descendants of Joseph.

Only in case of war were they to combine under the chieftainship of Judah. In the Book of Judges, however, we find the tribes as separate in matters of war as in peace; and by the time of Saul, the need for a closer union had been felt, and tribal independence had been found to lead only to anarchy.