Complex sentence: After immediate hostilities there had ended, Natori went to Japan and set up the first Nihon Kōbō, and when that collapsed set up the second, working on its magazine Nippon.
Split into simpler sentences: After immediate hostilities there had ended, Natori went to Japan and set up the first Nihon Kōbō. When that collapsed he set up the second, working on its magazine Nippon.