It is easy to become fatalistic about the eventual triumph of Islam throughout the world. Islamists really do believe their doctrine, whereas so many in the formerly Christian West view Christian doctrine as a fable.
But, historically at least, Islam has an Achilles Heel. It is that ever since Islam was first made up by Muhammad in the seventh century AD it has been a House of War. It is not just that the scimitar was deployed to spread the religion of "peace" to infidels; rather, intra-Islamic war has also been the norm. This is unlikely to change. As Efraim Karsh reminds us
Conflicts and wars among members of the same civilization have been far more common, and far more intense, than those between members of rival civilizations. Even in Islam, where Muhammad specifically and categorically forbade fighting among believers, it took a mere twenty-four hours after the Prophet's death for the head of the universal Islamic community, the caliph Uthman, to be murdered by political rivals. This opened the floodgates to incessant infighting within the House of Islam, which has not ceased to date and has exacted far more numerous casualties than conflicts between Muslims and non-Muslims. [Efraim Karsh, Islamic Imperialism: A History (New Haven: Yale University Press, 2007), p. 238.]We see this writ large in the Middle East today.
The deadly struggles between Sunni and Shi'a, and between those who side with the self-proclaimed Caliph Baghdadi and those who do not, and the deadly clash between the Kurds and the Turks are modern examples of Islam displaying itself to be the House of Internecine War.
These fatricidal wars are not minor skirmishes. Rather, they constantly tend towards genocide. For within the Islamic mindset, he who is not with us is against us. A member of another, rival sect is by definition a heretic or lapsed Muslim, worthy only of death. There seems little compunction against deploying poison gas, or other weapons of mass destruction against so-called Muslim brothers. Using women and children as human shields is no problem. Suicide bombers get a quick ticket to Paradise, and so forth.
Karsh reckons this is unlikely to change.
. . . the fact is that the fuel of Islamic imperialism remains as volatile as ever, and is very far from having burned itself out. Only when the political elites of the Middle East and the Muslim world reconcile themselves to the reality of state nationalism, forswear pan-Arab and pan- Islamic imperialist dreams, and make Islam a matter of private faith rather than a tool of political ambition will the inhabitants of these regions at least be able to look forward to a better future free of would-be Saladins. [Ibid., p. 240f.]We are bold enough to say that this will never happen while folk take Islam seriously as a belief system. The hadith (sayings) of the Prophet, the Koran itself, and Shariah law, which provide the fundamental authorities of the religion of Islam, require the use of force and the sword to defend the honour of Allah. This remains Islam's Achilles Heel, its greatest curse.