Scourge of War Boards

I'm writing a series about the Civil War, and I'm getting close to finishing the second book, (I'm sending the first in soon to an editor, wish me luck), and I'm doing the preliminary research for the 3rd book. It's going to be around the Second Manassas Campaign, and my dilemma is the Union Character. He is part of Richardson's Division, Sumner's Second Corps, which didn't participate in the fighting at Bull Run.

I'm trying to figure out some realistic reasons as to how he, or the regiment he is part, could be mixed in with Porter or Heintzelman's Corps just briefly enough so that he, or they, could be sent to fight. Any suggestions for reasons?

You could always give him the task of delivering a message to Pope asking what is going on but his horse gets shot out from under him by a rebel picket and he goes the rest of the way on foot but a sergent thinks he has left his regiment and tells him to get in line. So now your character is stuck in a regiment about ready to launch an attack on Hill's division.

Joshua l.Chamberlain wrote:

You could always give him the task of delivering a message to Pope asking what is going on but his horse gets shot out from under him by a rebel picket and he goes the rest of the way on foot but a sergent thinks he has left his regiment and tells him to get in line. So now your character is stuck in a regiment about ready to launch an attack on Hill's division.

I was thinking something along the lines of mistaken identify, as in one of Porter or Heintzelman's commanders thinks he's someone else. But that doesn't apply to the regiment as a whole, but if I have to, I'll keep them at Harrison's Landing