The Always War — Margaret Peterson Haddix

For as long as Tessa can remember, her country has been at war. When local golden boy Gideon Thrall is awarded a medal for courage, it's a rare bright spot for everyone in Tessa's town -- until Gideon refuses the award, claims he was a coward, and runs away. Tessa is bewildered, and can't help but follow Gideon to find out the truth. But Tessa is in for more than she bargained for. Before she knows it, she has stowed away on a rogue airplane and is headed for enemy territory. But all that pales when she discovers a shocking truth that rocks the foundation of everything she's ever believed -- a truth that will change the world. But is Tessa strong enough to bring it into the light?