Modern international Islamist terrorism is a natural offshoot of twentieth-century Islamic fundamentalism. The Islamic Movement emerged in the Arab world and British-ruled India as a response to the dismal state of Muslim society in those countries: social injustice, rejection of traditional mores, acceptance of foreign domination and culture. It perceives the malaise of modern Muslim societies as having strayed from the straight path (as-sirat al-mustaqim) and the solution to all ills in a return to the original mores of Islam. The problems addressed may be social or political: inequality, corruption, and oppression. But in traditional Islam and certainly in the worldview of the Islamic fundamentalist there is no separation between the political and the religious. Islam is, in essence, both religion and regime (din wa-dawla) and no area of human activity is outside its remit. Be the nature of the problem as it may, Islam is the solution.
The underlying element in the radical Islamist worldview is ahistoric and dichotomist: Perfection lies in the ways of the Prophet and the events of his time; therefore, religious innovations, philosophical relativism, and intellectual or political pluralism are anathema. In such a worldview, there can exist only two camps Dar al-Islam (The House of Islam i.e., the Muslim countries) and Dar al-Harb (The House of War i.e., countries ruled by any regime but Islam) which are pitted against each other until the final victory of Islam. These concepts are carried to their extreme conclusion by the radicals; however, they have deep roots in mainstream Islam.
While the trigger for Islamic awakening was frequently the meeting with the West, Islamic-motivated rebellions against colonial powers rarely involved individuals from other Muslim countries or broke out of the confines of the territories over which they were fighting. Until the 1980s, most fundamentalist movements such as the Muslim Brotherhood (Ikhwan Muslimun) were inward-looking; Western superiority was viewed as the result of Muslims having forsaken the teachings of the Prophet. Therefore, the remedy was, first, re-Islamization of Muslim society and restoration of an Islamic government, based on Islamic law (shariah). In this context, jihad was aimed mainly against apostate Muslim governments and societies, while the historic offensive jihad of the Muslim world against the infidels was put in abeyance (at least until the restoration of the caliphate).
Until the 1980s, attempts to mobilize Muslims all over the world for a jihad in one area of the world (Palestine, Kashmir) were unsuccessful. The Soviet invasion of Afghanistan was a watershed event, as it revived the concept of participation in jihad to evict an infidel occupier from a Muslim country as a personal duty (fard ein) for every capable Muslim. The basis of this duty derives from the irreversibility of Islamic identity both for individual Muslims (thus, capital punishment for apostates e.g., Salman Rushdie) and for Muslim territories. Therefore, any land (Afghanistan, Palestine, Kashmir, Chechnya, Spain) that had once been under the sway of Islamic law may not revert to control by any other law. In such a case, it becomes the personal duty of all Muslims in the land to fight a jihad to liberate it.
If they do not succeed, it becomes incumbent on any Muslim in a certain perimeter from that land to join the jihad and so forth. Accordingly, given the number of Muslim lands under infidel occupation and the length of time of those occupations, it is argued that it has become a personal duty for all Muslims to join the jihad. This duty if taken seriously is no less a religious imperative than the other five pillars of Islam (the statement of belief or shahadah, prayer, fasting, charity, and haj). It becomes a de facto (and in the eyes of some a de jure) sixth pillar; a Muslim who does not perform it will inherit hell.