I have just started playing the game seriously. In 1861 as the Union, I still can't create actual divisions/corps. I do have 5 early type 'divisions' and McDowell's Army HQ in Washington.
So starting with that I've tried having a First Mannasas battle two different ways:
1) I dragged all the divisions in Washington into McDowell's army. (That essentially destroyed the divisional organization - now all units are individually in the Army.) He was activated, I set the mode to offensive, marched to Manassas. There was a battle, with a Union overall loss.
2) I individually marched each of the divisions in Washington to Manassas, then marched Mcdowell to Manassas. Some of the division generals were not activated. So I guess that in this method, I essentially have 5 independent 'divisions' and a lone Army commander, with no troops in his Army. The resulting battle was very much like the first method, resulting in a a stalemate.
I guess my question is ... which of those methods should I have used in order to simulate First Mannassas? To me, the second method gives more of an historical flavor, and the divisions remain intact. However, in the second method McDowell was overall commander in name only.