You need to go out on a scouting mission and rescue Joseph Welsh. Sounds pretty easy, right?
Once you spawn, move down the hill and into the camp. Wait by the fireside for your orders. Seems that they think sending a small force into the swamp is a good idea. This can be easy or hard. I’ve seen the entire force get slaughtered and also seen a flawless victory.
You need to stick close to your allies for now. The Confederates in the swamp should fire at your allies just as often as they will at you. You can then trace the muzzle flash back to their position in the dark. The map should also list the nearby enemies.
Switch to the musket and move along slowly up to the lip of the swamp. Then start slowly moving across the center with the scouts. You should take this first fight as a good opportunity to perfect the quick reload for the musket. That will come in handy later.
You should meet some Confederates in the middle of the swamp. Stay low and carefully aim your shots. The pistol is a bit weak for the distance and the musket only has one shot. Search the bodies though. One of the first kills should have a Henry rifle lying beside it. Pick it up to make the rest of the fight much easier. A repeating rifle does wonders against these small pockets of resistance.