Murray Rothbard’s timeless essay is a must to read in its entirety. You can also find the PDF here.
The Conservative has long been marked, whether he knows it or not, by long-run pessimism: by the belief that the long-run trend, and therefore Time itself, is against him, and hence the inevitable trend runs toward left-wing statism at home and Communism abroad. It is this long-run despair that accounts for the Conservative’s rather bizarre short-run optimism; for since the long run is given up as hopeless, the Conservative feels that his only hope of success rests in the current moment. In foreign affairs, this point of view leads the Conservative to call for desperate showdowns with Communism, for he feels that the longer he waits the worse things will ineluctably become; at home, it leads him to total concentration on the very next election, where he is always hoping for victory and never achieving it. The quintessence of the Practical Man, and beset by long-run despair, the Conservative refuses to think or plan beyond the election of the day.
Pessimism, however, both short-run and long-run, is precisely what the prognosis of Conservatism deserves; for Conservatism is a dying remnant of the ancien régime of the preindustrial era, and, as such, it has no future. In its contemporary American form, the recent Conservative Revival embodied the death throes of an ineluctably moribund, Fundamentalist, rural, small-town, white Anglo-Saxon America. What, however, of the prospects for liberty? For too many libertarians mistakenly link the prognosis for liberty with that of the seemingly stronger and supposedly allied Conservative movement; this linkage makes the characteristic long-run pessimism of the modern libertarian easy to understand. But this paper contends that, while the short-run prospects for liberty at home and abroad may seem dim, the proper attitude for the libertarian to take is that of unquenchable long-run optimism.
The case for this assertion rests on a certain view of history: which holds, first, that before the 18th century in Western Europe there existed (and still continues to exist outside the West) an identifiable Old Order. Whether the Old Order took the form of feudalism or Oriental despotism, it was marked by tyranny, exploitation, stagnation, fixed caste, and hopelessness and starvation for the bulk of the population. In sum, life was “nasty, brutish, and short”; here was Maine’s “society of status” and Spencer’s “military society.” The ruling classes, or castes, governed by conquest and by getting the masses to believe in the alleged divine imprimatur to their rule.
The Old Order was, and still remains, the great and mighty enemy of liberty; and it was particularly mighty in the past because there was then no inevitability about its overthrow. When we consider that basically the Old Order had existed since the dawn of history, in all civilizations, we can appreciate even more the glory and the magnitude of the triumph of the liberal revolution of and around the 18th century.
Part of the dimensions of this struggle has been obscured by a great myth of the history of Western Europe implanted by antiliberal German historians of the late 19th century. The myth held that the growth of absolute monarchies and of mercantilism in the early modern era was necessary for the development of capitalism, since these served to liberate the merchants and the people from local feudal restrictions. In actuality, this was not at all the case; the King and his nation-State served rather as a superfeudal overlord re-imposing and reinforcing feudalism just as it was being dissolved by the peaceful growth of the market economy. The King superimposed his own restrictions and monopoly privileges onto those of the feudal regime. The absolute monarchs were the Old Order writ large and made even more despotic than before. Capitalism, indeed, flourished earliest and most actively precisely in those areas where the central State was weak or non-existent: the Italian cities, the Hanseatic League, the confederation of 17th century Holland. Finally, the old order was overthrown or severely shaken in its grip in two ways. One was by industry and the market expanding through the interstices of the feudal order (e.g., industry in England developing in the countryside beyond the grip of feudal, State, and guild restrictions.) More important was a series of cataclysmic revolutions that blasted loose the Old Order and the old ruling classes: the English Revolutions of the 17th century, the American Revolution, and the French Revolution, all of which were necessary to the ushering in of the Industrial Revolution and of at least partial victories for individual liberty, laissez-faire separation of church-and-state, and international peace. The society of status gave way, at least partially, to the “society of contract”; the military society gave way partially to the “industrial society.” The mass of the population now achieved a mobility of labor and place, and accelerating expansion of their living standards, for which they had scarcely dared to hope. Liberalism had indeed brought to the Western world not only liberty, the prospect of peace, and the rising living standards of an industrial society, but above all perhaps, it brought hope, a hope in ever-greater progress that lifted the mass of mankind out of its age-old sink of stagnation and despair.
Soon there developed in Western Europe two great political ideologies, centered around this new revolutionary phenomenon: the one was Liberalism, the party of hope, of radicalism, of liberty, of the Industrial Revolution, of progress, of humanity; the other was Conservatism, the party of reaction, the party that longed to restore the hierarchy, statism, theocracy, serfdom, and class exploitation of the old order. Since liberalism admittedly had reason on its side, the Conservatives darkened the ideological atmosphere with obscurantist calls for romanticism, tradition, theocracy, and irrationalism. Political ideologies were polarized, with Liberalism on the extreme “Left,” and Conservatism on the extreme “Right,” of the ideological spectrum. That genuine Liberalism was essentially radical and revolutionary was brilliantly perceived, in the twilight of its impact, by the great Lord Acton (one of the few figures in the history of thought who, charmingly, grew more radical as he grew older). Acton wrote that “Liberalism wishes for what ought to be, irrespective of what is.” In working out this view, incidentally, it was Acton, not Trotsky, who first arrived at the concept of the “permanent revolution.” As Gertrude Himmelfarb wrote, in her excellent study of Acton:
his philosophy develop(ed) to the point where the future was seen as the avowed enemy of the past, and where the past was allowed no authority except as it happened to conform to morality. To take seriously this Liberal theory of history, to give precedence to “what ought to be” over “what is,” was, he admitted, virtually to install a “revolution in permanence.”
The “revolution in permanence,” as Acton hinted in the inaugural lecture and admitted frankly in his notes, was the culmination of his philosophy of history and theory of politics… This idea of conscience, that men carry about with them the knowledge of good and evil, is the very root of revolution, for it destroys the sanctity of the past… “Liberalism is essentially revolutionary,” Acton observed. “Facts must yield to ideas. Peaceably and patiently if possible. Violently if not.” 
The Liberal, wrote Acton, far surpassed the Whig:
The Whig governed by compromise. The Liberal begins the reign of ideas… One is practical, gradual, ready for compromise. The other works out a principle philosophically. One is a policy aiming at a philosophy. The other is a philosophy seeking a policy. 
What happened to Liberalism? Why then did it decline during the nineteenth century? This question has been pondered many times, but perhaps the basic reason was an inner rot within the vitals of Liberalism itself. For, with the partial success of the Liberal Revolution in the West, the Liberals increasingly abandoned their radical fervor, and therefore their liberal goals, to rest content with a mere defense of the uninspiring and defective status quo. Two philosophical roots of this decay may be discerned: First, the abandonment of natural rights and “higher law” theory for utilitarianism. For only forms of natural or higher law theory can provide a radical base outside the existing system from which to challenge the status quo; and only such theory furnishes a sense of necessary immediacy to the libertarian struggle, by focussing on the necessity of bringing existing criminal rulers to the bar of justice. Utilitarians, on the other hand, in abandoning justice for expediency, also abandon immediacy for quiet stagnation and inevitably end up as objective apologists for the existing order.
The second great philosophical influence on the decline of Liberalism was evolutionism, or Social Darwinism, which put the finishing touches to Liberalism as a radical force in society. For the Social Darwinist erroneously saw history and society through the peaceful, rose-colored glasses of infinitely slow, infinitely gradual social evolution. Ignoring the prime fact that no ruling caste in history has ever voluntarily surrendered its power, and that therefore Liberalism had to break through by means of a series of revolutions, the Social Darwinists looked forward peacefully and cheerfully to thousands of years of infinitely gradual evolution to the next supposedly inevitable stage of individualism.