Take a story's temperature by studying its tone. Is it hopeful? Cynical? Snarky? Playful?
Sympathetic, Broad-minded
Wars are pretty much all about sides. Good vs. bad. Us vs. them. The Rebel Alliance vs. Stormtroopers. (Yep, even fictional wars are all about sides.)
But, despite the fact that this is a book about war, Myers doesn't take sides. The book paints a sympathetic picture of both the Iraqis, whose country is suddenly torn apart, and people like Birdy, who are soldiers trying to do their best in—understatement of the year time—a difficult situation.
Just check out the descriptions. It's all there, especially in the statements about what war has done to Birdy's mental state:
What I was supposed to do was chill, and I realized I was working on it way too hard. I kept reaching for the weapon I had left back in Baghdad. (14.193)
It's subtle, sure, but the way Birdy can't relax, even when he's basically on a resort compound, shows the effect the war is having on him. It might be years until he feels completely safe. None of the characters leave the war unaffected by it.