One of the most important things you can do is keep the Robbers Cave experiment in your mind at all time. Two groups of culturally homogeneous boys were slowly introduced to one another, and conflict blazed out of nowhere. Just the mere grouping caused the boys to detect minute differences and exaggerate them until two distinct cultures emerged, at odds and in constant competition. Only a tough common problem united them in a single struggle. Only after several more of these combined efforts did they decrease hostilities. Go! Go read it now!