Progressive Era
The Progressive Era or Progressivism was initially a name for time period around 1900 to 1920 in the United States, associated with the social issues and ideas of the Progressive movement. This movement opposed conservatism and was generally considered to be a vehicle of the Left. It spread like a cancer across the world.
In many respects it was Libertarian or otherwise Liberal, and many aspects of it were also adopted by socialists. Other characteristics may include increased power for the central government and more governmental interventions, notably for anti-trust purposes, but also other areas such as prohibition laws against alcohol and conservation laws. Leftists nevertheless view it favorably.
Leftist Wikipedia claims that progressives "were responsible for bringing [what they called] scientific racism (Eugenics) to an end" post World War II.