Readers who would like to learn a bit more about the background of the conflict between Ryn and the Straussians can find a reasonably good overview of historicism in UVA's e-text of the Dictionary of the History of Ideas.
The entry makes a point about the relationship of historicism (and its antecedents) to classical philology that deserves comment in light of the Strauss discussion:
[Classical art historian Johann Joachim] Winckelmann's approach foreshadowed the critical philological method which emerged among German classicists and Biblicists in the late eighteenth century and which laid the foundation for the critical historical method of nineteenth-century historical scholarship (Niebuhr, Ranke). This method has often been confused with the critical examination of evidence. The criteria for such examination had already been systematically formulated by Jean Mabillon in 1681 in his De re diplomatica. For F. A. Wolf, in his Prolegomena to Homer (1795), scholarship did not end with the critical analysis of documents but proceeded to view these documents in their historical context. Greek philology for him concerned itself with the totality of Greek life as reflected in the remnants of the Greek past, particularly its literature. Immersed in his evidence, the scholar then by inference and intuition had to proceed to the comprehension of the spirit of the culture in which the evidence originated. … The critical use of the documents required more than the establishment of a correct and authentic text, although the latter was the prerequisite of all scholarship. To be understood the documents had to be examined within the historical and cultural framework of the age and nation of which they formed a part.
We can see here one reason that relatively few latter-day Straussians are classicists: that last sentence is something that directly contradicts the Straussian creed. I suspect we can also see here whence the Straussians derive their enthusiasm for "literal translations" — to a mainstream philologist, there can be no such thing as a literal translation, since languages do not simply confer the same universal ideas in different words, but rather have words that stand for ideas that are particular to their cultures and not entirely analogous to the words (and meanings) of other languages. Philos is not quite the same thing as "friend"; eleutheria is not the same thing as "freedom" or libertas; and even within the same language, words can have different shades of meaning at different times (putting aside for a moment the basic reality that words can change their meanings over time: the point is rather that a word like "liberty" or "patriot" could have different shadings in 1776 as opposed to what it has today, even if the chief meaning of the term is the same or nearly the same). But if you believe that ideas are not very strongly historically conditioned, it follows that the ideas foreign words denote cannot be much different from the ideas that our words signify, and absolutely precise translation, or close to it, becomes possible.
Also worthy of note in light of the Strauss controversy is the German pedigree of historicism. Much of the criticism of and skepticism toward historicism, and not just on the part of the Straussians, comes from its historical connection with Germany. The idea is that by justifying German particularism — the nationalism of der Volk — against universal, rational laws and norms, historicism eroded restraints on tribalistic politics and helped lead the way to the persecution of the Jews. There has also been a conjunction — whether contingent or necessary is debatable — between historicism and mystification of the State as embodying the nation; ein Volk, ein Reich, as it were, asserting their historical individuality and independence against the universal constructs of other nations, which universalisms are not really universal at all, but rather the historical expressions of particular peoples, with the corollary that the imposition of those universalisms on (or even their voluntary acceptance by) another people constitutes conquest. Conversely, military strength can also translate into cultural strength (and as some would have it, different nations and their cultures are always in competition with one another, and if a nation-culture is not expanding, it is necessarily weakening or being conquered):
For Hegel, as for Ranke, the state was an expression of spirituality; but for Hegel the spirituality of the state rested on its rational structure and the course of history itself was the test of its rationality. For Ranke the test of its spirituality and the justification of its power-political striving was its uniqueness which defied all rational inquiry or judgment. … Ranke uncritically assumed that all power rests on spiritual foundations, that the state in extending its military power strengthens the foundations of freedom and culture. Nevertheless, such a view introduced metaphysical assumptions as well as an ideological bias into historical writing. It subordinated all considerations of domestic policy and social concern to the strivings of the established political order to maintain its power.
Ranke was no proto-Nazi, nor were historicists like Meinecke (who was dismissed from an academic post under the Nazis and for whom Goethe, not Hitler, was the true expression of the German spirit) actual Nazis. Still less does any of this implicate the historicism of Claes Ryn. But one can see why Leo Strauss, a Jewish refugee, might have been inclined to view historicism in the worst possible light.