Karl Popper's rejection of Marxism during his teenage years left a profound mark on his thought. He had at one point joined a socialist association, and for a few months in 1919 considered himself a communist. During this time he became familiar with the Marxist view of economics, class-war, and history. Although he quickly became disillusioned with the views expounded by Marxism, his flirtation with the ideology led him to distance himself from those who believed that spilling blood for the sake of a revolution was necessary. He came to realise that when it came to sacrificing human lives, one was to think and act with extreme prudence.
When did Popper think it was unnecessary to act with prudence? 