In 1918, Okawa joined the South Manchurian Railway Company. He then earned his doctorate at Tokyo University and taught the history of colonialism at Takushoku University. He proposed that Japan liberate Asia from western imperialism. Okawa founded nationalistic organizations such as Yuzon-sha that were designed to preserve traditional Japanese values and promote the renovation of Japan.
In the first half of the 1930s, Okawa established connections with likeminded military officers and became involved in plans for a coup d'état. In 1932, Okawa was arrested for his involvement in the May 15 Incident, the assassination of Prime Minister Inukai Tsuyoshi. In October 1935, he was sentenced to five years in prison, but he was released as a part of general amnesty after serving less than two years.
Following his release, Okawa published numerous works on the history of Japan and on western colonialism and expansionism in Asia. He advocated the necessity for Asian nations to cooperate and adopt common policies against the west. During World War II, Okawa argued for "Asia for the Asians" and called on all Asian peoples to join Japan in the "Great Asian War" against the Allied powers.
Following World War II, the Allied powers viewed Okawa's "Asia for Asians" policy as the foundation for Japan's Greater East Asia Co-prosperity Sphere that had led to the Pacific war. He was arrested as a Class A war criminal but was declared unfit to stand trial by reason of insanity. Okawa recovered in two years and continued to publish and promote his ideology until his death in Tokyo on 24 December 1957. Asakawa Michio
Okawa Shumei. Okawa Shumei shu [Works of Shumei Okawa]. Edited by Bunzo Hashikawa. Tokyo: Chikuma Shobo, 1975.