One important factor in O'Brien's The Things They Carried the idea of the collective as opposed to the individual. In the military environment, the characters' survival depends on their ability to function as a unit, collectively. However, the things they carried in their backpacks reveal each as an individual person. Each has his own passions, regrets, and sense of loss. In O'Brien's work, military discipline is not so much consciously imposed as it is the result of collective functioning.
To look at this a little more closely, one might consider how leadership works at the outpost. The appointed leader is Lieutenant Jimmy Cross. Cross, however, appears to be less than confident about his leadership skills, especially when Ted Lavender is killed on his way back from relieving himself. As such, he is more part of the collective entity that is the platoon than its acting leader.
Nevertheless, the collective discipline works well for the team. Nobody, for example, questions the way they function, because everybody understands his duty. This duty is to expect others to risk their lives for the protection of the outpost, while each individual is also willing to risk his own life for the protection of others. Hence, when they draw lots to identify a person to inspect the tunnels, nobody questions his duty when the lot falls on him.
Collective discipline at the outpost is therefore connected to the loyalty, dependence, and sense of duty among the platoon members.