In 1944, just as mankind plunged into total war, an enemy called Genjuu appeared from another dimension. As soon as they appeared, they waged a campaign of indiscriminate annihilation against mankind. Like phantoms, they appeared out of nowhere and began the massacre, and then faded away like phantoms. Despite mankind possessing the most combat potential ever in its history, they were still repeatedly defeated by the Genjuus and lost much territory. There is a periodicity to the Genjuu's appearance, and despite lying in wait with perfect preparations there had been no success against them. In the following 50 years, mankind continued to lose battles, human lives, territory, and dignity. Genjuu literally means "phantom beasts".