Tristram is now a Sergeant in the British Army, and his duties are to instruct lower-ranking soldiers in elementary literacy, arithmetic, report-writing, and other practical skills.
Tristram is brought before a Lieutenant-Colonel, who accuses him of teaching the men inappropriately—to whit, urging them to ask questions like "Who Is the Enemy?" and "What's All The Fighting About?" (5.2.8), you know, dumb useless questions like that.
As punishment, Tristram is relieved of his instructional duties, and transferred to a rifle company that will soon be moving out to the front.
Tristram is mostly confused: as far as he knows, there's no real front to speak of, because England isn't actually at war.
When Tristram meets up with his new company, he and another Lieutenant track down their Battalion Orders. To their surprise, they really are heading into action soon.