The morning after they arrive at the base camp, Tristram and the others get word that they'll be heading to the front that evening.
At twilight, the men march out, and eventually make their way to the trench where they'll set up and wait for the enemy's attack.
Tristram tries to convince the others that the whole war is a fake, and that soon the trench will simply be blown up. He's realized that this must be the government's new way of "dealing with excess population" (5.7.17).
When Tristram tries to walk away, his commanding officer fires a warning shot at him. Tristram gives up, and decides to stay.