During 1945, while serving as a chief of the Kaigun Bukanfu (the Japanese Navy's Communications Office) under Admiral Tadashi Maeda, Yoshizumi secretly aligned with Indonesian nationalism. According to statements by his friend, Shigetada Nishijima, he had actually adopted Marxism, having previously been a supporter of right-wing Japanese nationalism. On 16 August 1945, prominent nationalist leaders Sukarno and Muhammad Hatta were kidnapped and brought to Rengasdengklok, Karawang by some other figures in the movement to be convinced to declare Indonesian independence, and afterwards Yoshizumi was sent to accompany them back, to ensure that the military would not interfere. Later, he would help to compose the text of the proclamation itself. Yoshizumi met Tan Malaka in Achmad Soebardjo's house shortly after the proclamation. He then deserted his post and aligned with Indonesia when he joined with Tan Malaka's group, where he was called Arif. He arrived there with stolen Japanese Navy resources, which went into funding Tan Malaka's guerilla movement.