...which must surely be the island setting for this World War II -based novel: Pianosa. After all, the novel's context entirely fits the set-up given for this puzzle (a small island somewhere in Europe, [on which] a battle is fought in the middle of World War I)!
It's worth noting also that the puzzle's instructions have been written in a way that imply that to find the flag you need to find the hidden values, whilst findingto find the hidden values willit might help you to find the flag. This paradox is very reminiscent of the so-called 'logic' at the heart of the 'Catch-22' in the novel, in which the book's fighter pilots will only be excused from flying dangerous missions if they are found to be insane, but any pilot who expresses a concern that they might be insane is thereby showing themselves to be sane by demonstrating a rational concern for their own safety, and thus cannot be exempted from flying! (Yes, it's a head-scratcher...)