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I had the opposite interpretation.
The origin of the war was purposely unexplained, so as to highlight how immaterial the reasons for war are to the ordinary people caught up in it, whose lives are ruined and transformed beyond recognition by it.
Also, by removing the back story the film is able to transport the people of middle America into a war such as exists in Somalia, Sudan or any number of "foreign" places.
I found this device very effective.
It's like someone gave $100m to some kids to make a movie. There's no story. Nothing about how or why there'd be such an improbable alliance between the 3 states. It's just a series of combat set pieces. And with all that money they didn't even choreograph the combat properly. Hard to believe that Alex Garland could make something so bad.