Popper’s most fundamental criticism of Marx is expressed in this paragraph:

Why, then, attack Marx? In spite of his merits, Marx was, I believe, a false prophet. He was a prophet of the course of history, and his prophecies did not come true; but this is not my main accusation. It is much more important that he misled scores of intelligent people into believing that historical prophecy is the scientific way of approaching social problems. Marx is responsible for the devastating influence of the historicist method of thought within the ranks of those who wish to advance the cause of an open society.

Essentially, what Popper has in mind is that historicists think that the entire historical process has a final goal or outcome. People cannot change this process; it cannot be accelerated or altered. According to Marx’s version of historicism, the final goal of the historical process is socialism, but this cannot be achieved until all the productive possibilities of the previous stage—capitalism—have been realized. It would be a serious mistake to try to establish socialism before this, and the attempt to do so would necessarily fail.