Sarah Manavis in New Statesman:
Cultural Marxism is a theory that started in the early 20th century, which was popularised in the aftermath of the socialist revolution (this great piece in the Guardian explains it in depth). The idea was that Marxism should extend beyond class and into cultural equality and that, through major institutions like schools and the media, cultural values could progressively be changed. The theory was later adopted by the philosophers at the Frankfurt School who posited that the only way to destroy capitalism was to destroy it in all walks of life; where, not just classes, but all genders, races, and religions could live in society equally.
While this may seem unimportant, the Frankfurt School’s adoption of – and modifications to – cultural Marxism is where the conspiracy theory truly begins. The Frankfurt School’s predominantly Jewish members of the school were forced to flee to America by the Nazis in the 1940s, where many went on to teach, write, and commentate in mainstream institutions. This, conspiracy theorists claim, is when cultural Marxists began to poison the West – and when cultural Marxists began their attempts to undermine its values.
More here.