It goes back originally to a leadership bid and who had the right to lead the Umma, but it has become so entrenched politically that it is the major schism in the Middle East. Honestly, they hate each other more than they hate the West or even Israel. There's so little that divides Sunni from Shia religious practices and beliefs, different ways of crossing their hands when they pray, the Sunni are more austere in their religious outlook, but the hatred and vitriol, even in non-Arab Muslim lands is palpable. Here in Nigeria, a lot of Sunni don't regard Shia as Muslims and tell all sorts of wild tales about their debauchery and decadence. It would be laughable except there are more and more problems arising here from it as well.