Why do we need a "tribe" to survive? This is such a silly idea. The world has become. and will continue to become a much smaller place. Doing stupid **** like putting up a Trump wall will only make the inevitable more painful. But I guess if "invasion" is a word you like to throw around then I assume you are a Trumpite.
I wasn't referring to "invasions" by caravans & migrants, I'm talking about literally survival, so attacks & invasions by foreign countries and their militaries, like say Russia or whomever. If the world didn't have countries with borders that protected themselves from outside violent aggressors, all you would need is one moderately powerful group (tribe) to form and grab all the territory for themselves.
A tribe in the form of a country also has other security benefits, like ability to negotiate trade deals etc that benefit the inhabitants, and negotiate alliances with other "tribes" so they can collectively protect each other from foreign military aggression.