I'm trying to think of mass-appeal war stories with a balanced or ambiguous or at least non-stupid treatment of good/bad

The saga of A Song of Ice and Fire has sold around 7 million copies (Wikipedia) and it's extremely far away from Manichean morality. I would estimate that no more than five percent of the text involves truly heroic or truly depraved characters.

Sven Hassel's best-selling books can also be a good example. We must, however, distinguish between works that derive their nuanced morality from an attempt to be faithful to reality, and those that donate nuanced morality to a fictional setting.