He’s written a useful discussion of the collapse of liberalism into radical-left extremism: Supremely Modern Liberals.
Hitchcock seems to waffle somewhat on the nature of liberalism itself and whether things might have turned out otherwise. That might be because he’s a historian and as such tends to view something like liberalism as a complex historical formation rather than a thing with a definite nature that can be described and reasoned about. Or maybe it’s because all Americans except a few cranks are basically liberals, and he’s writing as usual for an audience and publication (in this case Touchstone) that wants to avoid crankishness and aims at a supposedly solid middle ground of orthodox reasonableness.
My view, of course, is that anything recognizable as liberalism represents a decision to take freedom rather than the good as the final standard for resolving disputes—in other words, to put human will before God—and once that choice has been made everything else follows. The saying from the 30s that communists are liberals in a hurry is therefore essentially correct. The opposing neoconservative (and libertarian) view, that liberalism could have kept chugging along forever but got derailed for some special reason that can be reversed, is, I believe, the illusion that makes conservative efforts forever useless. So the nature of liberalism, however metaphysical the question might seem, is an important issue.