Here is the set-up provided in the trailer, translated by Gematsu...
The lifeline of the Empire of Japan was likely to collapse.
The country was devastated by the repeated defeat and frequent battles in the Great East Asian War, which spanned over 100 years.
Now, most of the country is occupied by enemy nations, and American and Soviet Union military forces are approaching the Imperial Capital, Tokyo.
It was clear to everyone that the country’s downfall was just a matter of time.