The experience of living under Japan’s fascist government and American bombing of Tokyo would shape a lifelong opposition to war, especially nuclear arms, and imperialism.

In the immediate postwar period, Katō joined a Japanese-American research team to assess the effects of the atomic bombing of Hiroshima.

After participating in a 1958 conference of writers from Asia and Africa, he gave up practicing medicine entirely.

While being deeply focused on Japanese culture and classical Chinese literature, Katō gained a reputation for examining Japan through both domestic and foreign perspectives.

In 2004, he formed a group with philosopher Shunsuke Tsurumi and novelist Kenzaburō Ōe to defend the war-renouncing Article 9 of the Constitution of Japan.