Here is G.K. Chesterton on Islam, with his usual prophetic insight. The names of the 19th and early 20th century Islamic militants may not be familiar except to a few students of history, but we are all too familiar with their spiritual descendents:

"There is in Islam a paradox which is perhaps a permanent menace. The great creed born in the desert creates a kind of ecstasy out of the very emptiness of its own land, and even, one may say, out of the emptiness of its own theology. It affirms, with no little sublimity, something that is not merely the singleness but rather the solitude of God. There is the same extreme simplification in the solitary figure of the Prophet; and yet this isolation perpetually reacts into its own opposite. A void is made in the heart of Islam which has to be filled up again and again by a mere repetition of the revolution that founded it. There are no sacraments; the only thing that can happen is a sort of apocalypse, as unique as the end of the world; so the apocalypse can only be repeated and the world end again and again. There are no priests; and yet this equality can only breed a multitude of lawless prophets almost as numerous as priests. The very dogma that there is only one Mahomet produces an endless procession of Mahomets. Of these the mightiest in modern times were the man whose name was Ahmed, and whose more famous title was the Mahdi; and his more ferocious successor Abdullahi, who was generally known as the Khalifa. These great fanatics, or great creators of fanaticism, succeeded in making a militarism almost as famous and formidable as that of the Turkish Empire on whose frontiers it hovered, and in spreading a reign of terror such as can seldom be organised except by civilisation…"