If you want to see a Catholic historian implement some of Franz Oppenheimer's findings (purposefully, I don't know), check out Christopher Dawson's essay, "Warrior Peoples and the Decline of Archaic Civilization" (found in Dynamics of World History), in which he points out the warrior class "was always in a sense parasitic" on the settled pastoral society around it.
I have no idea whether Dawson agreed with Oppenheimer, but I read a lot of Dawson as a younger man. He was easily the historian I read the most of, unless you put Voegelin under the category of "historian." If Dawson's historical conclusions correspond well to the libertarian's, I'm wondering if I internalized a lot of Dawson's ideas, with the result that, 20 years later, the historical worldview of the libertarians resonated with me.
Addition to academic bucket list: Go back and read Dawson.
