Ever since the election of President Donald J. Trump, a lot of the discussion on the left has tended to revolve around the concept of "identity politics", where more traditional leftists seek a return to the issues of class, economics, foreign policy and war, whereas the identitarian left seeks to refocus the discussion on gender and sexual orientation, race, and culture.
In the 20th century, a group of German intellectuals, now known as the Frankfurt school, sought to apply Marx and Hegel's dialectal, critical method to subjects other than political economy and history. Using insights from Freudian psychology, existentialism, and other sources, they created a philosophy known as "Critical Theory".
This theory gained popularity in the 1960's and 70's as the disillusion with overzealous abuses of power in actually existing Marxist-Leninist states such as the Soviet Union, opened up a space for movements like the New Left. During the same time, a new related philosophy called postmodernism was beginning to take off as well. Postmodernism rejected the “Modernism” of 19th and 20th century western philosophy. According to the postmodern philosophers, many western philosophies inspired by enlightenment rationality, contained ‘meta-narratives’, or ‘grand, master stories’ about history like the notion of progress, or Marxist historical materialism.
It is worth noting here that any discussion of these topics tends to be lumped in the anti-Semitic “Cultural Marxism” conspiracy theory, which tends to characterize the Frankfurt school’s work as a ‘Jewish plot’ to destroy white western civilization, and ascribes an influence and important to the Frankfurt school far beyond that which it actually had in left and center-left movements, thought, and realpolitik. Another objection to this is that the classification of the Frankfurt School theorists as ‘Marxists’ is incorrect, as many have pointed out that Adorno, Horkheimer, etc. did not subscribe to many of the basic tenets of Marxism, such as value or exploitation theory, or historical materialism.
For more on this, see Council Communist blogger Michael Acuna’s essay “The Origins and Ideological Function of Cultural Marxism” at: commonruin.files.wordpress.com
But it wasn't really until the late 80's and early 90's, that scholars (mostly women of color) applied critical theory to race and gender issues in a new formulation called "Critical Race Theory". This is where the idea of 'intersectionality' comes from.