Sorry. I thought I was explaining away.
Try this:
Webster's Dictionary, definition 1, "tribe": " a group of persons, families or clans descended from a common ancestor and forming, together with their slaves, adopted strangers, etc. , a community."
Definition 3: "Any primitive or nomadic group of people of generally common ancestry, possessing common leadership.'
Intermarriage, community and leadership are the essential elements. Some religions function, or at least under certain circumstances function, more as tribes than do others. The Mormons were setting up a theocratic government in the state of Deseret when they were stopped by the US federal government. The organization itself is still very much there; Mormons still think of themselves as a people apart, and are tribal according to the above definitions.
The Amish are another people apart. They speak either antiquated English or an equally antiquated German. Their religious language is German. They have special dispensation from the government to leave school after the eighth grade and do not serve in the armed forces. They do not normally marry outside their own faith and are largely internally self-governing. This makes them tribal.
The Irish, Scots and Welch have nation-states, by definition tribal.
The individual Somali clans are tribes in the same senses: they share genetic makeup, have a common leadership and form a community. They are separate nation states because ( at least in the perceived male line) they do not share any of the essential elements that would allow them to unite under a tribal structure. That's why it has to be federation for the long haul.