One of the more depressing facts of human existence is the need of groups for enemies. Now that Republicans and conservatives dominate practically every facet of American government, we see them turning on each other. This is not a new trend.
Fredrick S. Starr, writing about Shia vs Sunni in the Muslim world and Central Asia in particular, in the Tenth Century, CE:
Strange to say, this growing fractiousness among Muslims arose just as Islam was becoming the majority faith throughout the region. Many adherents of other religions in territories conquered by the Arabs had converted or emigrated. Others had been successfully marginalized, while still others had made their peace with the Muslim order.
Starr, S. Frederick. Lost Enlightenment: Central Asia's Golden Age from the Arab Conquest to Tamerlane (p. 242). Princeton University Press. Kindle Edition.
Actually it's not strange at all, if you think about the theory of evolution. Competition for survival of individuals promotes the formation of groups and competition among them.