By Robert Spencer (from "Pure Islam" and Michael van der Galiën)
Shafi'i school: A Shafi'i manual of Islamic law that was certified in 1991 by the clerics at Al-Azhar University, one of the leading authorities in the Islamic world, as a reliable guide to Sunni orthodoxy, stipulates that “the caliph makes war upon Jews, Christians, and Zoroastrians...until they become Muslim or pay the non-Muslim poll tax.” It adds a comment by Sheikh Nuh ‘Ali Salman, a Jordanian expert on Islamic jurisprudence: the caliph wages this war only “provided that he has first invited [Jews, Christians, and Zoroastrians] to enter Islam in faith and practice, and if they will not, then invited them to enter the social order of Islam by paying the non-Muslim poll tax (jizya)...while remaining in their ancestral religions.” ('Umdat al-Salik, o9.8).
Of course, there is no caliph today, and hence the oft-repeated claim that Osama et al are waging jihad illegitimately, as no state authority has authorized their jihad. But they explain their actions in terms of defensive jihad, which needs no state authority to call it, and becomes "obligatory for everyone" ('Umdat al-Salik, o9.3) if a Muslim land is attacked. The end of the defensive jihad, however, is not peaceful coexistence with non-Muslims as equals: 'Umdat al-Salik specifies that the warfare against non-Muslims must continue until "the final descent of Jesus." After that, "nothing but Islam will be accepted from them, for taking the poll tax is only effective until Jesus' descent" (o9.8).
Hanafi school: A Hanafi manual of Islamic law repeats the same injunctions. It insists that people must be called to embrace Islam before being fought, “because the Prophet so instructed his commanders, directing them to call the infidels to the faith.” It emphasizes that jihad must not be waged for economic gain, but solely for religious reasons: from the call to Islam “the people will hence perceive that they are attacked for the sake of religion, and not for the sake of taking their property, or making slaves of their children, and on this consideration it is possible that they may be induced to agree to the call, in order to save themselves from the troubles of war.”
However, “if the infidels, upon receiving the call, neither consent to it nor agree to pay capitation tax [jizya], it is then incumbent on the Muslims to call upon God for assistance, and to make war upon them, because God is the assistant of those who serve Him, and the destroyer of His enemies, the infidels, and it is necessary to implore His aid upon every occasion; the Prophet, moreover, commands us so to do.” (Al-Hidayah, II.140)
Maliki school: Ibn Khaldun (1332-1406), a pioneering historian and philosopher, was also a Maliki legal theorist. In his renowned Muqaddimah, the first work of historical theory, he notes that “in the Muslim community, the holy war is a religious duty, because of the universalism of the Muslim mission and (the obligation to) convert everybody to Islam either by persuasion or by force.” In Islam, the person in charge of religious affairs is concerned with “power politics,” because Islam is “under obligation to gain power over other nations.”
Hanbali school: The great medieval theorist of what is commonly known today as radical or fundamentalist Islam, Ibn Taymiyya (Taqi al-Din Ahmad Ibn Taymiyya, 1263-1328), was a Hanbali jurist. He directed that “since lawful warfare is essentially jihad and since its aim is that the religion is God’s entirely and God’s word is uppermost, therefore according to all Muslims, those who stand in the way of this aim must be fought.”