In Algeria, though Islamic fundamentalist terrorists are engaged in indiscriminate killing, including foreigners, most of the killings are done by Muslims to other Muslims. On a broader Muslim canvas, there are also the divisions between the Shi'a and the Sunni sects. Intra-Christian rivalries can be illustrated by the example of Protestants versus Catholics in Northern Ireland cited above, but also by that of Christian Orthodox Serbs versus Catholic Croats, and many others. Tensions between Jew and Jew are not far below the surface, especially between Ashkenazi Jews from Eastern Europe and Sephardic Jews from the Mediterranean region. Religious communities therefore may often have a common external enemy, but what is also remarkable is the degree to which internal enemies are generated in virtually all major religions, most prominently in the monotheistic ones.
Religion is just one cause of communitarian strife.
Ethnicity, race, language, or simply the destiny of history are other powerful causes. The resentments, tensions, hostilities, and violence found in one form or another in most countries are products of history. Blacks resent whites in the United States not simply because of discrimination, but also because of the great injustice of history, the slave trade. Countless variations exist on historical myth and historical grudge, and on what events or forces may stand out in the collective memory of any particular community. Just over two hundred years ago the natives of the Vendee fought a civil war against the forces of the Revolution.