This is a solid movie about young men at war. The monsters are a surreal backdrop, they're what the soldiers think they're there to fight, and they do, but mostly they fight other people, and the monsters are just also there. Theres a lot of metaphor there, I think it's best for the viewer to make their own conclusions about that.
It's a gritty, realistic war movie, it's meant to make you think. Not a typical gung-ho style flick where the good guys always win.