I had the honor of meeting Fritz Wilhelmsen at a seminar at Steubenville in 1990 on, yes, the thought of John Courtney Murray. During a break I asked him if the red berets they wore that day had anything to do with Carlism (I already knew that he taught a course each year at the Escorial). He drew himself up to an imposing height (he was taller than me, anyway) and said: "Why of course. I am an honorary Requeté you know." He was a legendary character.
His daughter, Dr. Alexandra Wilhelmsen, has actually pubished some very interesting articles on Carlism, pointing out some of its more attractive features as mentioned in part in this thread. To me it was especially interesting to find that Carlism was a monarchist movement that recognized that a monarch could in effect abandon his right to the throne by failing in certain respects in doing his duty, such as to uphold the fueros, as I recall it. "Pure" monarchists would say that the king could never abandon his rights, even through abdication--I think this is the Habsburg family's position, in effect denying the validity of the abdication by Blessed Charles of Austria.
One or two Carlist pretenders when making an incursion into Spain swore the royal oath to uphold the fueros (I believe this was done even at the famed tree of Guernica, where the old rulers had sworn the oath and which was what made Guernica significant in the first place, but which the Nazis were too clueless to know to destroy in their air raid).
This whole subject is part of a much larger historical topic that seems largely unknown today, the extent of local and regional liberties in pre-Revolutionary Europe. To juxtapose the thoughts here, I first had my consciousness raised on this point by an Opus Dei priest and himself famous University of Navarre historian, Fr. Federico Suarez, writing in a festschrift book for Fritz ("Saints, Sovereigns and Scholars" [I may have reversed the order, though]), in which he noted in passing that monarchical central governments interfered less in the daily lives of the people than post-Rveolutionary governments do. His cite was to a very interesting book by Frantz Funck-Brentano, entitled something like "The Ancien Regime in France." That book is itself very worthwhile reading, and really eye-opening in a way. If so moved, some day I will cite passages from it.
Fritz wrote articles for the SSPX's "Angelus" magazine in his later years, and Alexandra told me that when she told him that he would be thought to have gone over to the Lefebvrists (he in fact was an adherent of the FSSP's Indult Mass in Dallas), he told her "most people ecumenize to the left, but I ecumenize to the right."
A couple of corrections and additions:
The Funck-Brentano book is entitled "The Old Regime in France".
Fr. Suarez's contribution to "Saints, Sovereigns and Scholars" (R. Herrera, J. Lehrberger and M. E. Bradford, eds., New York, 1993) is entitled "The Influence of the French Revolution on the Political Configuration of Europe". As part of the introduction to his article, he says the following:
"[A] few facts should suffice as indicators to help form an idea of what the 'absolutism' of the Ancien Regime actually was. In the age of Philip II of Spain, a delinquent who had committed a crime in the Kingdom of Valencia had only to cross the border to the neighboring Kingdom of Murcia to live in peace, because he had not broken the law there. No authority in any one of the Spanish realms had power in another; and, of course, the Cortes or parliaments of the kingdoms tended to grant subsidies only if the monarch previously had committed himself to give satisfaction to the grievances brought before him. In France, in 1621, Louis XIII and his minister Richelieu considered establishing certain customs at the Pyrenees to tax Spanish merchandise. Languedoc objected, and both the king and his minister had to bow their heads and give up the project.
"Compare these limitations of the faculties of the monarch in the period of full absolutism to the powers of parliaments (and heads of government) in the regimes born of the Revolution, and perhaps we can see, with a bit of reflection, which of these two political systems can best be classified as governmental 'absolutism'" (Id. pp. 323-24 [trans. A. Wilhelmsen]).
Note: the age of Philip II was after the unification of Spain under Ferdinand and Isabella, so Fr. Suarez is describing the situation in the "unified" Spain.
A final note: Fr. Suarez published more than 20 volumes of academic history in Spain, but he is best known in the English speaking world for his religious books, including "Mary of Nazareth" and "Joseph of Nazareth" (both Scepter Press), which are among, if not the, best Catholic books on their subjects. As a good son of St Josemaria, he died in his late 80's having just completed his latest book--working with order and intensity to the end.