If there is one common thread that connects President-elect Donald Trump’s foreign-policy appointments, it is the belief that the United States is engaged in a civilizational war against “radical Islamic terrorism.” Indeed, while Trump himself—with his ambivalence towards books—may not even know it, the incoming administration seems to have fully embraced the ideas of Samuel Huntington, not only on the clash of civilizations, but on American decline, the idea of a West encircled by enemies, and even on immigration. Throughout the campaign, Trump and his surrogates often used rhetoric that...