Story is based in japan sci-fi edo/era, with the American invaders portrayed as the 'Amanto' in the series (essentially alien like monsters, it's ironic Asians in reality have this common view that foreigners are all aliens)
From the start it seems the country is at war with the Amanto, but later on in the series I beg to question whether the characters were actually at war with themselves....
(not a true synopsis of the entire show, but don't want to be spoiling anything)