Due to the long distances and Allied naval dominance, cooperation between Japan and the European Axis powers during World War II was slight. However, there was some limited exchange of materials and personnel. Early on, this was achieved with disguised cargo ships. Later, the task was turned over to submarines, though this dramatically limited the amount of cargo that could be transported. The only submarine to successfully make the round trip was the Japanese I-8, which departed Japan 1 June, 1943, reached France on 31 August, and spent the next month there while her crew became Axis propaganda celebrities. I-8 headed for Japan on 5 October, with a host of German tech, and several VIPs. She returned to Japan on 21 December.
The other four Japanese submarines that attempted the voyage were all sunk en route, and the lone U-boat that made it to Japan, U-511, joined the IJN.