Deported from Java following the start of World War II, Yoshizumi returned illegally and was arrested in 1941, when Japan declared war on the Netherlands. He spent time in an Australian internment camp, but was released during a prisoner exchange in August 1942, and returned to the East Indies, which were by then under Japanese occupation. Although serving as head of the Japanese intelligence board Kaigun Bukanfu, by 1945 he embraced Indonesian nationalism and Marxism. He ensured that Sukarno and Muhammad Hatta signed their names to the Proclamation of Indonesian Independence, which he also helped write. Yoshizumi then joined Tan Malaka's network of anti-Dutch guerillas, taking part as a soldier in the clashes of East Java. These activities inaugurated his participation in the National Revolution, which ended with his death from lung disease in 1948.