He worked for the Kempeitai (military secret police) at the construction of the Burma Railway in Thailand, and spent most of his later life as an activist for post-war reconciliation and against Japanese militarism.

He made over a hundred visits to Thailand, and from the 1970s, arranged several meetings between former Allied prisoners of wars and their Japanese captors, in efforts to promote peace and understanding.

Following Japan's surrender, Nagase spent seven weeks working for the Allied War Graves Commission as a volunteer, helping recover bodies for proper burial.

Nagase was first introduced to the British public in the documentary made by ex-POW John Coast about the realities of life on the Thai-Burma Railway, which was first broadcast in the UK on BBC2 on 15 March 1969.

[5] Nagase is portrayed by Randall Duk Kim in the 1996 BBC TV film Prisoners in Time, based on the story of Eric Lomax, who is played by John Hurt, with Rowena Cooper as his wife, Patti.