I was searching about alternate histories where Japan is divided like Korea post WWII, and I found one entry in Wikipedia. It's called Seito: Victory Road, a novel by Daisuke Sato. He's the same writer who penned Highschool of the Dead. Yes, the zombie manga.
The premise of the story is that the Battle of Midway was won by Japan, but not only still lost the war but also had the Soviets take over Northern Hokkaido as well! Southern Sakhalin remained (North) Japanese and thus the Democratic People's Republic of Japan was born.