The preachers of a new ideology have to decide how they are going to react to the traditional beliefs and institutions of their society. Are they going to annihilate all traces of tradition in a brutal revolutionary war and then create a new utopia, or are they going to let the forces of tradition live and, through a long drawn process of compromise and coercion (carrot and stick), try to transform or destroy those aspects of tradition that they dislike?
The preachers of leftism, in the first half of the twentieth century, opted for the first model—they decided that they would cleanse their society of all traditional beliefs and institutions and then commence the work for the creation of a communist utopia. But that led to mass slaughters and extreme poverty and starvation in the Soviet Union. Between the 1930s and 1960s, it dawned on the leftists that the cost of annihilating the traditional beliefs and institutions is too high and that the mindless destruction caused by Lenin, Trotsky, and Stalin in the Soviet Union was doing nothing to further the communist cause.
After the 1930s, the leftists started moving towards the model of compromising with the existing traditions—this gave rise to the intellectual movements like Fabian Socialism and Frankfurt School and the political movements like liberalism and progressivism which became very popular and are currently ruling most of the democratic nations in the world. What the brutal tactics of communism could not achieve, the soft tactics of liberalism and progressivism has.