The period after WW I was that of the weakening of the laissez faire capitalism. Virtually every state started introducing social safety net. In some countries, like Sweden, it led to a generaous welfare state and stoped short of nationalzing key industries (although such proposals were entertained). In the US, it did not go that far. The forces behind those changes were labor unions, workers parties, progressive intellectuals etc. - and how much they could achive depended mainly on the strength of the capitalist reaction to those changes. Some of those changes were later reversed as the capitalist class regained strenght and launched a reactionary movement.
That is a historical narrative that gives justice to power struggle. However, if we take the "system as a whole" approach and start seeing all what is going as something that feeds into the system - we abandon the realm of empirical science and enter the realm of teleological mysticism. Which is what much of modern Marxism cum world system are. Basically, the same genre as Talcott Parsons' social systems and neo-classical economics, but with with reversed polarities.
Wojtek