Leftism in the Western World has roots reaching way back into the dim past. Leftist ideas and notions made themselves felt again and again in late medieval and modern history, but for its first concrete and, in a way, fateful outbreak and concretization we have to look to the French Revolution.
That's the Cliff's Notes version….
My intent is to focus on the time beginning with the European Middle Ages. My reasons for not reaching back any further in time should be clear to regular readers; also, I think it will be clear from some of what is offered by EvKL. For example, regarding the political liberties of the Greece of Plato and Aristotle:
…while social liberties were perhaps marked, political liberties were few, though here we have to bear in mind that the concept of the person as we know it did not exist in antiquity. It makes its appearance in the Western World-and solely in the Western World-only with the advent of Christianity.