Answer:
infantry tactics
Explanation:
Soldiers were drilled in infantry tactics, usually based upon a manual written before the war by West Point professor William J. Hardee (Rifle and Light Infantry Tactics: for the Instruction, Exercise and Maneuver of Riflemen and Light Infantry, published in 1855).
I forgot what happened. A bunch of things happened.
D is the correct answer. Hope I could help.
A,B and the one you already have checked