He was a prominent figure in radical politics in Japan, opposing the Russo-Japanese War by founding the Heimin-sha group and its associated newspaper, "Heimin Shinbun". Due to disregard for state press laws, the newspaper ceased publication in January 1905, and Kōtoku served five months in prison from February to July 1905. He subsequently left for the United States, spending November 1905 until June 1906 largely in California, and he came into contact with other prominent anarchist figures such as Peter Kropotkin.