Deneen's overall thesis is that all liberalism contains within itself the seeds of its own destruction--progressive liberalism may get to the bottom of the slippery slope faster, but classical liberalism or libertarianism will get to the bottom just as surely because their fundamental principles are the same. Indeed, in a notable quote (see below) Deneen states with regard to the historical ignorance of his students:
The pervasive ignorance of our students ... is the consequence of a civilizational commitment to civilizational suicide.
The civilization he speaks of, of course, is that of Western liberalism.
Deneen approaches the question of liberalism's death-wish from a philosophical and historical perspective but, before dismissing this as arcane theorizing, be advised that Deneen--writing in 2018--is keenly aware of current political realities. He writes with Trump--and "populism" generally--very much at the front of his mind.
To give a preliminary flavor of where Deneen is coming from, here is an excerpt from his Wikipedia page:
Deneen's interests have ranged in the areas of ancient and American political thought, democratic theory, political theology, literature and politics, and political economy. He is a noted "Catholic communitarian," a traditionalist whose conservatism leads him to break with many American political conservative positions and liberal orthodoxies, including a critique of "laissez faire" market economics and a defense of environmentalism and sustainable economic practices, a stress upon more local forms of production and exchange, combined with a critique of progressive sexual liberationism and a suspicion toward technology that advances these basic liberal commitments. Among his influences are the democratic communitarianism of his teacher, Wilson Carey McWilliams; liberal democracy's "friendly critic," Alexis de Tocqueville; the social historian and critic of modern liberalism—especially its tendencies to bifurcate elite from populace—Christopher Lasch; and the agrarian essayist, novelist and poet Wendell Berry. Deneen's work has long manifested a critical stance toward liberalism for its tendencies toward individualism, its hostility to culture and tradition, its undermining of religious belief, its depersonalization and fragmentation of social solidarity, its hostility toward nature, family, and community. These critiques—evident in academic and more popular writings throughout the late 1990s and 2000s, culminated in the publication of his book Why Liberalism Failed (Yale, 2018).
Deneen has lamented the loss of a knowledge of the history of western civilization among his students:
My students ... are the culmination of western civilization, a civilization that has forgotten nearly everything about itself, and as a result, has achieved near-perfect indifference to its own culture... The pervasive ignorance of our students is not a mere accident or unfortunate but correctible outcome, if only we hire better teachers or tweak the reading lists in high school. It is the consequence of a civilizational commitment to civilizational suicide. The end of history for our students signals the End of History for the West.
What I intend to do is present--in four parts--the preface to Deneen's paperback edition, in which he summarizes the argument of the book. However, I'll start with an extended quote that Deneen places at the beginning of the book. The quote is from a famous book by Barbara Tuchman, concerning the collapse of the medieval synthesis beginning in the 14th century--a prelude to the rise of liberalism and the fracturing of Christendom. Clearly Deneen, by placing this quote at the beginning of his book, wants us to draw the comparison between the collapse of medieval Christendom and the current state of collapse of our own constitutional order.
The gap between medieval Christianity's ruling principle and everyday life is the great pitfall of the Middle Ages. It is the problem that runs through Gibbon's history, which he dealt with by a delicately malicious levity, pricking at every turn what seemed to him the hypocrisy of the Christian ideal as opposed to natural human functioning. ...
Chivalry, the dominant idea of the ruling class, left as great a gap between ideal and practice as religion. The ideal was a vision of order maintained by the warrior class and formulated in the image of the round Table, nature's perfect shape. King Arthur's knights adventured for the right against dragons, enchanters, and wicked men, establishing order in a wild world. So their living counterparts were supposed, in theory, to serve as defenders of the Faith, upholders of justice, champions of the oppressed. In practice, there were themselves the oppressors, and by the 14th century the violence and lawlessness of men of the sword had become a major agency of disorder. When the gap between ideal and real becomes too wide, the system breaks down. Legend and story have always reflected this; in the Arthurian romances the Round Table is shattered from within. The sword is returned to the lake; the effort begins anew. Violent, destructive, greedy, fallible as he may be, man retains his vision of order and resumes his search.
--Barbara Tuchman, A Distant Mirror; The Calamitous 14th Century
Clearly, Deneen wants us to relate this to modern life in America. Our ideal is a vision of constitutional order--but what has become of it? Nine blackrobes attempting--all too often--to impose their preferences on the nation. Certainly in the age of Trump we see a judiciary out of control--in "resistance" to the very constitutional order they're sworn to uphold. They have become the oppressors. Deneen wants us to consider: Is this some bizarre accident, or is it the working out of the very principles behind liberalism?