Politically speaking, it's probably the most explosive suggestion you can make today: that the Left has joined hands with radical Islam. That it is fellow-traveling with it. Such a suggestion will get you branded a McCarthyite, immediately. But is it true (the suggestion, that is)? Afraid so. And this case is powerfully, sickeningly made in David Horowitz's new book.

At first blush, it may seem an odd alliance: the leftists and the Islamists. After all, Islamists are premodern "conservatives." Reflecting on a big anti-war rally in London, Mark Steyn pointed out that militant lesbians were marching alongside militant Muslims. Did the former care that the latter would have them dead? Not really.

What unites the Left and Islamism, above all, is a deep-seated hatred of the United States (and, secondarily, Israel). Also, an absolutist, totalist view of the world. Those are enough. . . .

The author of this book is known as a hothead, a "flamethrower"; indeed, when Horowitz plays at his keyboard, fireworks can result. But this is a coolly argued book. It is eloquent, unrelenting — devastating. It records what has occurred thus far, and explains why it has occurred. Horowitz may be as valuable to us today as his ex-radical forebears — many of them associated with this magazine [National Review] — were in their own day. Horowitz utterly understands the War on Terror and its opponents, in all their flavors. Unholy Alliance is, in fact, a weapon in this war.