"...Exempted from automatic execution, it appears, are Christians who do not resist their new government. Baghdadi permits them to live, as long as they pay a special tax, known as the jizya, and acknowledge their subjugation. The Koranic authority for this practice is not in dispute....<snip>....
The Koran specifies crucifixion as one of the only punishments permitted for enemies of Islam. The tax on Christians finds clear endorsement in the Surah Al-Tawba, the Koran’s ninth chapter, which instructs Muslims to fight Christians and Jews “until they pay the jizya with willing submission, and feel themselves subdued.” The Prophet, whom all Muslims consider exemplary, imposed these rules and owned slaves....."
If al-Qaeda wanted to revive slavery, it never said so. And why would it? Silence on slavery probably reflected strategic thinking, with public sympathies in mind: when the Islamic State began enslaving people, even some of its supporters balked. Nonetheless, the caliphate has continued to embrace slavery and crucifixion without apology. “We will conquer your Rome, break your crosses, and enslave your women,” Adnani, the spokesman, promised in one of his periodic valentines to the West. “If we do not reach that time, then our children and grandchildren will reach it, and they will sell your sons as slaves at the slave market.”
In October, Dabiq, the magazine of the Islamic State, published “The Revival of Slavery Before the Hour,” an article that took up the question of whether Yazidis (the members of an ancient Kurdish sect that borrows elements of Islam, and had come under attack from Islamic State forces in northern Iraq) are lapsed Muslims, and therefore marked for death, or merely pagans and therefore fair game for enslavement. A study group of Islamic State scholars had convened, on government orders, to resolve this issue. If they are pagans, the article’s anonymous author wrote,
Yazidi women and children [are to be] divided according to the Shariah amongst the fighters of the Islamic State who participated in the Sinjar operations [in northern Iraq] … Enslaving the families of the kuffar [infidels] and taking their women as concubines is a firmly established aspect of the Shariah that if one were to deny or mock, he would be denying or mocking the verses of the Koran and the narrations of the Prophet … and thereby apostatizing from Islam.
What ISIS Really Wants - The Atlantic