Yoshimoto, who had pursued a theory of war responsibility of the literati, supported the
Anpo Protests against the 1960 revision of the
US-Japan Security Treaty as an expression of the contradictions of the postwar order fifteen years after the end of the war. Strongly opposing the new treaty, he became an "enthusiastic supporter" and "patron saint" of the
Zengakuren student activists. Yoshimoto was invited to give speeches at Zengakuren meetings in December 1959 and January 1960, and he joined the student activists in a sit-in at
Shinagawa Station in
Tokyo as part of a nationwide general strike against the Treaty on June 4, 1960. In the aftermath, Yoshimoto was arrested and interrogated for three days by the police, before being released without charges. Yoshimoto concluded that the only path forward was to reject the oppression of existence and pursue absolute individual autonomy (
jiritsusei). New Left activists especially appreciated that Yoshimoto was developing a positive theoretical discourse in the midst of the collapse of the Communist Party's heroic status after the failure of the anti-Treaty movement and endless, contentious and dispiriting schisms within the left. Yoshimoto's books became best-sellers, especially his 1962 essay collection
The End of Fictions, named after his famous 1960 essay of the same name (which was anthologized within). In these and other essays, Yoshimoto developed an independent theory of the arts in the face of criticisms of the Communist Party and sectarian literary theories, emphasizing the aesthetics of language and psychological phenomena, and his concept of "communal fantasy" (共同幻想,
kyōdō gensō), describing how the propaganda and militarism of the wartime era "swept away virtually the entire population in a wave of war frenzy". Yoshimoto's philosophy of radical individualism became a refuge for students and intellectuals exasperated by the then-current sectarian and bureaucratic Marxism. As a result, Yoshimoto's anti-sectarian philosophy of independence and individualism became a major influence and theoretical resource in the 1960s and 1970s for the Zengakuren,
Zenkyoto, and other 'non-sect' New Leftists. He was regarded as required reading for participants in the
1968–69 Japanese university protests. This was in spite of his remaining aloof from, and taking a critical stance toward, the student protests of the later 1960s, a stance which was a consequence of his aversion to sectarianism and party-driven movements. Yoshimoto eventually concluded that even the radically egalitarian and highly individualistic New Left protest groups were not individualistic enough, and were still part of the same form of "communal fantasy" (共同幻想,
kyōdō gensō) which had led Japan into
World War II. ==From the 1980s==