Chapter 3 is titled "Prisoner of War." In this chapter, we learn about how Vladek was drafted into the army and how he became a soldier.
As he describes it, Vladek did not shoot at the beginning of the battle for two reasons. First, there did not appear to be any targets. No one was shooting at him, and therefore, he didn't see any reason to fire his weapon. Secondly, war was still a new experience to the newly conscripted Vladek. So he wanted to be sure before he discharged the contents of his weapon.
Eventually, Vladek changed his thoughts about shooting, after he caught sight of a moving tree. He concluded that he should fire, since trees are never known to move of their own accord. After he fired off a bullet, a hand rose from the tree to indicate a desire to surrender. Despite this, Vladek continued shooting until the tree stopped moving. He believed that his actions were necessary because he had no way of knowing whether the enemy soldier (behind the tree) would fire his own weapon.
Apparently, Vladek had always wanted to join the army. His father, however, had not been so enthusiastic about any of his sons fighting. In fact, Vladek's father put him on a starvation diet so that he would fail the required army medical examinations. For three months, Vladek ate nothing but salted herring. He was also not allowed to sleep for more than three hours a night. Yet, the army medics declared him healthy. They asked him to build himself up and to report again the next year.
Upon hearing this, Vladek's father pressured him to go back on the same starvation diet. However, Vladek had had enough of starving. In the end, he joined the army.