Captain America: The Ghost Army by Alan Gratz

In this thrilling historical adventure, 18-year-old Steve Rogers (AKA Captain America) and his young sidekick, Bucky Barnes are fighting in WWII when they encounter a threat like none they’ve ever seen — a Ghost Army. The dead of this war and wars past are coming back to life, impervious to bullets, flames, or anything else the Allies can throw at them. The armies rise from the ground in the night and seem to disappear without a trace. How can Cap and Buck fight something that’s already dead? And just what does the mysterious Baron Mordo, sitting in his castle atop nearby Wundagore Mountain have to do with this?