The Young Lieutenant by Oliver Optic

Probably there was no one in either party who was so thoroughly bewildered by the incident which had just transpired as Captain Somers. The mystery of his companion's antecedents was in a fair way to be cleared up, though in a very unsatisfactory manner to those most intimately concerned. The conversation, and the verification of the rebel officer's statements, showed that De Banyan was not De Banyan; that the brave and brilliant Federal officer was not a Federal officer; that, of all he had been, only the "brave" and "brilliant" remained.

It was painfully evident that the bold and dashing captain was, or had been, a rebel officer. Somers was terribly shocked at the discovery, even while it was a satisfaction to have the mystery of his companion's previous life explained. For the time, he forgot the perils of his own situation in the interest he felt in the affairs of his friend. Perhaps De Banyan was a spy, who had been serving in the Union army for the purpose of conveying information to the enemy. He had been very glad of the opportunity to cross the river; and it seemed probable to our hero that he wished to return to his friends. It is true, the efficient services of the captain in the Army of the Potomac, his readiness at all times to fight the rebels, and especially his shooting down the enemy's pickets in the swamp, were not exactly consistent with such a record; but perhaps he had done these things to keep up appearances, and thus enable him the better to promote the objects of the rebellion.