The Huffington Post:
By Ronald Tiersky, Eastman professor of political science, Amherst College
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 worshiped.