Two minutes after his message had been received Private Brown, white-faced and haggard, was placed under arrest.
Under grilling, he confessed what Secret Service men had already learned---that his name was really spelled B-r-a-u-n; that both he and his father were German subjects, and that the young man had enlisted for the sole purpose of playing the spy and the plotter in the Army.
It had been Mock's talk of deserting in France that had caused Braun to talk to Mock, who had been told by Captain Prescott to talk in that vein while in the bull-pen.

Braun had fallen into the trap.
As for Wilhelm---which wasn't the young an's real name---he was the son of a German-born father, but a young man of known loyalty to the United States.

He wasn't a soldier, but a War Department agent who had donned the uniform for a purpose, and had come to Camp Berry with a draft of real soldiers.
And this was the plan that Dick had worked out following his pretended arrest of Mock that night up the road.

Mock, resolved to become a good soldier again, had undergone his humiliation in the bull-pen, and the scorn of his fellow-prisoners, in order to trap the stoop-shouldered German, a pretended carpenter, but really August Biederfeld, a German spy.

The bespectacled one, Dr.Carl Ebers, was another spy.