Today's posting on Brian's blog mentions a wonderful Antietam campaign map project he's working on--daily operational maps from September 4-20. I am impressed with the detail and the beauty of the maps and have a deep appreciation for this comment of his:
As I have been plotting units, though, I’m finding there are far more of them than I’d read of previously, and the neat orders of battle with which I’m familiar are of less and less help. Local militia, orphaned cavalry detachments, signalmen, state troops; not to mention rear-area logistics, supply, and communications points.
The OBs of most campaign and battle histories (even the very good ones) include only the formations present on the "main" subject battlefield, generally ignoring those-- both large and small--that played even significant operational roles at some point in the campaign leading up to the climactic battle.