Robert Spencer talks about the great Islamic scholar Majid Khadduri, dead this year at age 98, in "Fact or 'Islamophobia'?". Khadduri, an Iraqi scholar of Islamic law, helpfully explained the doctrine of jihad in his 1955 book, "War and Peace in the Law of Islam."
"The state which is regarded as the instrument for universalizing a certain religion must perforce be an ever expanding state. The Islamic state, whose principal function was to put God’s law into practice, sought to establish Islam as the dominant reigning ideology over the entire world. It refused to recognize the coexistence of non-Muslim communities, except perhaps as subordinate entities, because by its very nature a universal state tolerates the existence of no other state than itself. Although it was not a consciously formulated policy, Muhammad’s early successors, after Islam became supreme in Arabia, were determined to embark on a ceaseless war of conquest in the name of Islam. The jihad was therefore employed as an instrument for both the universalization of religion and the establishment of an imperial world state. (P. 51)In other words, jihad is not something acted out by a minority of Islamic extremists contrary to the "Religion of Peace." Jihad against non-Muslims is a duty for all Muslims until they conquer the world. In other words, religious slaughter is part of Islam's DNA.
"Thus the jihad may be regarded as Islam’s instrument or carrying out its ultimate objective by turning all people into believers, if not in the prophethood of Muhammad (as in the case of the dhimmis), at least in the belief in God. The Prophet Muhammad is reported to have declared “some of my people will continue to fight victoriously for the sake of the truth until the last one of them will combat the anti-Christ.” Until that moment is reached the jihad, in one form or another, will remain as a permanent obligation upon the entire Muslim community. It follows that the existence of a dar al-harb is ultimately outlawed under the Islamic jural order; that the dar al-Islam is permanently under jihad obligation until the dar al-harb is reduced to non-existence; and that any community which prefers to remain non-Islamic -- in the status of a tolerated religious community accepting certain disabilities -- must submit to Islamic rule and reside in the dar al-Islam or be bound as clients to the Muslim community. (Page 64)"