Just how long has the war been going on? Or maybe it's an endless series of wars.
Ishiyumi tends to show flashbacks a lot, which this pic is. In the current timeline the war is officially over and the world is in the process of rebuilding, but there are still a need for mercenaries (probably to fight roving bands of bandits and the like) and the horrors of the war are still roaming about (the "Apparitions"), which is why most people are armed.