Michael Beschlosis, writing for the NYT's The Upshot blog:
This is one of the earliest photographs ever taken of a baseball game, and it happened by accident. The photographer, Henry P. Moore, of Concord, N.H., was focusing on the well-uniformed Union soldiers of the 48th New York State Volunteer Infantry, but he also captured their baseball-playing comrades in the background.
By combining the Civil War and baseball, Moore’s photograph merges two of the most important elements of the American historical experience, both of which, to this day, have deep emotional resonance. Baseball did not became the “national pastime” until long after Appomattox, but Americans came to feel so passionately about the game that some mythmakers tried to embellish the historical record by exaggerating its importance during the time the Union fought the South.
Baseball as we know it didn't exist then; it was a completely different game.