TRIBALISM !!!
Every one belongs to a tribe - And its SO easy to get your own tribe - Look at teenage gangs - Tribalism - Look at the problems in Africa - Tribalism - Europeans drew lines on maps to decide which country owned which bit of territory regardless of who lived where , and where the Tribal boundaries were .
So YOUR tribe has 'this' distinction - Of Language , Accent , Birth , Eye Colour , or what ever . Even Post Code these days (Zip Code for our American Tribes )
And any one who is not YOUR tribe , is your enemy .

Some of us have progressed away from the simple tribalism , but too many havent , and that's where the trouble starts .