Kinda crazy how everyone immediately jumped at comparing this to WW2 and Nazis. This manga seems much more like WW1 with its pointlessness and intra-country genocides, and honestly even that's not a great comparison.
The muskets look Napoleonic, a lot of the designs look late Russian Empire and Prussian, and the warfare looks early 19th century. Plus the story so far seems like partisans fighting against their own government before fleeing to a hostile neighbor not currently at war.
TL;DR: This manga is a mix of several different time periods and peoples, cause it's building it's own settting. Maybe there will be more parallels with the World Wars later on, but right now it's just an interesting premise that needs time to breathe and develop.