First he helped lead America's Trotskyites out of the wilderness of the revolutionary communist left and into the Democratic, and then Republican, parties as the "neoconservative" movement.
Later, that movement plunged an ice axe into the skull of whatever residual revolutionary libertarian impulse may have remained alive in American conservatism by the Age of Nixon. As Kristol himself wrote in "The Neoconservative Persuasion" --
Neocons do not like the concentration of services in the welfare state and are happy to study alternative ways of delivering these services. But they are impatient with the Hayekian notion that we are on "the road to serfdom." Neocons do not feel that kind of alarm or anxiety about the growth of the state in the past century, seeing it as natural, indeed inevitable. Because they tend to be more interested in history than economics or sociology, they know that the 19th-century idea, so neatly propounded by Herbert Spencer in his "The Man Versus the State," was a historical eccentricity.
I do not generally rejoice on the occasion of anyone's death. The only thing I find worthy of mourning in the case of Kristol's, however, is the fact that it came far too late. America would likely be a better, freer nation today had he -- and the embryo of his pernicious ideology with him -- fallen in 1944 at Herrlisheim where he served with the 12th Armored Division.