The political columnist and cultural critic Mark Steyn has written a remarkable book, "America Alone: The End of the World as We Know It" (Regnery). He combines several virtues not commonly found together — humor, accurate reportage, and deep thinking — and then applies them to what is arguably the most consequential issue of our time: the Islamist threat to the West. Mr. Steyn offers a devastating thesis but presents it in bits and pieces, so I shall pull it together here. He begins with the legacy of two totalitarianisms. Traumatized by the electoral appeal of fascism, post-World War II...