As ISIS/Islamic State takes over Mosul and other parts of Syria and Iraq it is imposing an ancient religious persecution of minorities: Christians (or any minority) face a brutal choice of conversion to Islam, paying tribute, or death. Almost all have fled the city or the country. Jews throughout history have often faced a similar choice: convert or suffer tragic consequences. Some converted officially but practiced their Judaism in secret. Early Christians were persecuted by the Roman Empire before triumphing when the Emperor Constantine himself became a Christian. Christians and Muslims warred on each other for two centuries over control of the Holy Land. Religious Wars in 16th/17th century Europe pitted Catholics and Protestants in murderous combat.
The justification is always the same: intransigent belief about whose God is the One True God and how this God must be worshipped.
In ancient pagan cultures, such as Greece and Rome, there were many gods each with limited powers who coexisted in an imagined world of conflicts and delights. Religious war in our sense was unthinkable precisely because there were many gods, none of which was the One True God. In war and conquest the gods of other peoples could be accepted and/or manipulated because the goal was dominion rather than religious truth. (Alexander the Great practiced this to perfection.)
The potential for religious war was transformed by the advent of monotheism, the belief that there is only One Almighty God that characterizes the three Abrahamic faiths: Judaism, Christianity and Islam. Monotheism was a great civilizing discovery but there is a dark side to the belief in One True God, which is escalation of the stakes in religious differences.
Whose God is the One True God? The Jewish Ten Commandments put it this way: "I am the Lord your God...you shall have no other gods before me." Christianity proclaims 'God the Father and Jesus Christ as his Only Begotten Son.' In Islam it is Allah: "There is no god but God and Mohammed is his Prophet." Is the One True God Yawveh, Christian God the Father or Muslim Allah? One might easily conclude that the three One True Gods are really versions of the same God whether God is believed to be a reality or a myth. That three monotheistic faiths emerged is thus an accident of history, what happened rather than what had to be. Gandhi said he was 'a Muslim, a Hindu, a Christian and a Jew'. The small Baha'i faith that began as an offshoot of Islam proposes that the various prophets from Abraham to their own Baha'u'llah are not in conflict but a progressive revelation of God's truth that will ultimately result in the Unity of All Mankind. For the moment, and for a thousand years, the world has been stuck with three monotheisms. (Buddhism, animism and other spiritual beliefs are of a different nature).
In the clash of monotheistic faiths, after two thousand years Judaism and Christianity have by and large made peace with each other. Much of Islam has done likewise but certain narrow-minded, violent versions of Islam, an example of which is ISIS/Islamic State, believe Almighty Allah demands submission of all Muslims to a central authority (a Caliphate that also abolishes internal Muslim differences such as Sunni and Shi'a), and a single form of Islamic society (sharia law). Universal supremacy is the ultimate goal of Islam. Remarkably, in today's Islamist wars in the Great Middle East and Central Asia the main battle is not between united Muslims against Jews and Christians but an internal sectarian struggle, mainly Sunni wars of fanaticism against Shi'a.
All institutions of civilization, including religions, are a mix of positive and negative effects. The State promotes welfare but limits freedom. Religion preaches love and community but also incompatibilities and conquest. The family is the primary human home but abuse within it is widespread. Civilization is simultaneously the evolution of human well-being and its burden.
Monotheism historically is an archetypal example of this duality. On one hand it creates unity; on the other it creates conflict. It expands the community (the "other" becomes one of "us".) but it also polarizes the passions it creates (their god is not the true god). There is no true way but our way.
The obvious resolution of the dilemma of monotheism is cosmopolitan, simple, logical and imperative: mutual tolerance of incompatible truths and the separation of religion and the State. Judaism, to its credit, never espoused conquest of the world (historically, even conversion was resisted). Christianity gradually gave up the idea of a universal Christian civilization. The mass of Islam tends today in the same direction but small parts of it are still fighting the trend of history. Approaching God, or rejecting the very idea (atheism), ought to be a personal matter, something like happiness as defined in the Declaration of Independence: a pursuit by each in their own way.
Religious fanaticism may eventually give way to the lesson of History. However there will likely be still more tragedy along the way.