I am afraid you are mistaken. We are hard wired to want to belong to a group. What that group is, is completely coincidental. Back to your football example, rival football fans become the best allies in the world cup. The us and them theory you purport is not the real reasons why people affiliate to a group and depending on the personality traits of each person, they might decide to join a confrontational group or one that seeks to co-operate with others for a common goal.