I have a particular interest in two (in certain respects antithetical) movements which flourished in Europe in the 1930s. Both had profound effects on subsequent history.
One was logical positivism, which developed in Vienna in the 1920s under the leadership of Moritz Schlick. The goal of the movement was to apply the latest discoveries and methods in physics, mathematics and logic to philosophical questions and, in so doing, to undermine metaphysics by showing its traditional problems to be pseudo-problems which had not been recognized as such by philosophers because of a naive approach to language. Ordinary language was not only vague, its grammar was permeated by metaphysical assumptions. Nietzsche had recognized this, but it was only the development of formal logic by Frege, Russell and others that allowed the deficiencies of ordinary language to be understood and ultimately to be left behind (so far at least as scientific activity was concerned).
The other major movement to which I refer is the European neo-liberal movement which crystallized for the first time at a Paris gathering in 1938, but which did not gain prestige and importance until after the War (notably in the form of the Mont Pelerin Society). This movement brought together economists and others who were convinced of the importance of competitive markets and who were resolutely opposed to totalitarianisms of the left and of the right.
What I find particularly curious is that the logical positivists (with a very few exceptions) were either socialists or social democrats. The only exceptions that I know of were foreign associates: Willard Van Orman Quine, an American in his twenties on a traveling scholarship, was certainly a conservative in his later years; and Louis Rougier, the preeminent French associate of the Vienna Circle, was a liberal conservative and, so far as I know, the only figure to be associated with both logical positivism and the European neo-liberal movement. (Indeed, Rougier played a leading role in the early years of the latter movement, and convened the 1938 Paris meeting.)